Chicken_Champ already quoted it above so I didn't feel the need to - but this is the sentence I meant:

If a character uses a ranged weapon to make a melee attack, or throws a melee weapon that does not have the thrown property, it also deals 1d4 damage.

It doesn't say the ranged weapon becomes an improvised weapon - though you could assume it implies that given that it's in the improvised weapons rule - but it doesn't say it explicitly - and rules only do what they say they do - nothing more.

I absolutely agree that a ranged weapon when used in melee is improvised and shouldn't be able to benefit from the 3rd bullet point of Sharpshooter (even with Tavern Brawler). But that's not how it's written.

I think the intention of Sharpshooter's third bullet point is that it is a ranged atrack with the ranged weapon, but yes it doesn't specify.

Personally, the fact that a ranged weapon melee attack both is recorded in the Improvised Weapon section and is given the same damage as generic improvised weapons, means that the ranged weapon is an improvised weapon when used for a melee strike so you don't have proficiency.

The issue, Bronzed, is that nothing in the Improvised Weapon Section ever explicitly says that melee attacks using ranged weapons lose proficiency bonus. For ease of reference on a new page, I'll quote it again from: PHB Chapter 5.

Sometimes characters don't have their weapons and have to attack with whatever is at hand. An improvised weapon includes any object you can wield in one or two hands, such as broken glass, a table leg, a frying pan, a wagon wheel, or a dead goblin.

Often, an improvised weapon is similar to an actual weapon and can be treated as such. For example, a table leg is akin to a club. At the DM's option, a character proficient with a weapon can use a similar object as if it were that weapon and use his or her proficiency bonus.

An object that bears no resemblance to a weapon deals 1d4 damage (the DM assigns a damage type appropriate to the object). If a character uses a ranged weapon to make a melee attack, or throws a melee weapon that does not have the thrown property, it also deals 1d4 damage. An improvised thrown weapon has a normal range of 20 feet and a long range of 60 feet.

The simplified takeaway that you, me, and everyone else takes from this is probably something like: "improvised weapon attacks deal 1d4 damage and don't use your proficiency bonus, unless the DM considers it to be similar enough to an actual weapon you're proficient with to be treated as such for purposes of Proficiency Bonus, Damage Die, Weapon Properties, and other features." The problem is... that really isn't what the section says!

• Nothing says that a ranged weapon used to make a melee attack, or a non-thrown melee weapon used to make a ranged attack, ceases to be a ranged/melee weapon and instead becomes a new type of "improvised weapon" or anything like that. Instead, all it says is that the weapon does d4 damage instead of its normal damage when used that way. 
• Nothing even says that a ranged weapon used to make a melee attack, or a melee weapon thrown to make a ranged attack, ceases to use its proficiency bonus.

Assumptions that the "improvised weapon" referred to in Tavern Brawler includes real simple/martial weapons used in a way that violates their normal type is just that: an assumption, not supported by the actual language in the Chapter 5 section. That section never calls these "improvised weapon attacks," or includes them in the list of example "improvised weapons" at the start of the section. This might be a good RAI assumption, especially since it's talked about in the "Improvised Weapons" section, but the RAW doesn't quite get there.... a Longbow is still a Longbow when you hit someone with it, and by RAW would seem to still be a martial ranged weapon that adds your Proficiency Bonus to attacks if you're proficient with it, with only the damage die changing to a d4.

To drag this back to the subject of Nets, this interpretation does a lot to rehabilitate them for characters that don't have Crossbow Expert or Sharpshooter, since it allows you to use a Net in melee without penalty as an improvised melee attack with a martial ranged weapon, using Strength instead of Dexterity and without Disadvantage. :) 

"At the DM's option, a character proficient with a weapon can use a similar object as if it were that weapon and use his or her proficiency bonus."

This is the only guidance for which ability mod you would be using for an attack with an Improvised Weapon. So, if you make a melee weapon attack with a Ranged Weapon that you are proficient with, do you use Dex (RAW, Ranged weapons use Dex) or Str (RAW, Melee weappns without Finesse use Str)

Because your proficiency with the weapon, is using it in the manner it is used in, the damage die, type and mod listed with the weapon, not your use of it according to the Improvised Weapons rule.

Quote from BurnishedBronze >>

"At the DM's option, a character proficient with a weapon can use a similar object as if it were that weapon and use his or her proficiency bonus."

This is the only guidance for which ability mod you would be using for an attack with an Improvised Weapon. So, if you make a melee weapon attack with a Ranged Weapon that you are proficient with, do you use Dex (RAW, Ranged weapons use Dex) or Str (RAW, Melee weappns without Finesse use Str)

Because your proficiency with the weapon, is using it in the manner it is used in, the damage die, type and mod listed with the weapon, not your use of it according to the Improvised Weapons rule.

 No, because Chapter 9 helpfully tells us that melee attacks always = Strength and ranged attacks always = Dexterity, unless a feature or weapon property allows you (or requires you, for Thrown) to use a different ability. It isn't that "melee weapons" use strength, it is that "melee weapon attacks" do. Improvised melee attacks with ranged weapons thus are required to use Strength, unless the weapon has the Finesse property or you have a class or other feature that allows you otherwise swap out your ability of choice. 

You are correct that Chapter 9 does specify what abilities to use. But I'm not convinced that you add your proficency to an attack made with the Improvised Weapons rule, without proficency in improvised weapons.

The fact that ranged weapons aren't explicitly mentioned in the list of examples of improvised weapons doesn't mean that they aren't an Improvised Weapon when used for a melee attack. The rule allowing you to make a melee attack with them is the Improvised Weapons rule.

Edit: If the rule allowing a melee attack with a ranged weapon was not part of the Improvised Weapon Rule, I would 100% agree with you

Edit Edit: I checked Sage Advice because I can form the words properly to explain:

https://www.sageadvice.eu/2018/02/26/can-you-use-a-longbow-in-melee-to-get-great-weapon-master-and-sharpshooter-in-a-single-attack-for-20-damage/

To quote here for ease:

If you use a weapon in a way that turns it into an improvised weapon—such as smacking someone with a bow—that weapon has none of its regular properties, unless the DM rules otherwise.

That tweet has him saying two things:

1. A Longbow used to whack someone is not a Longbow for that attack, it is an "Improvised Weapon"
2. The Improvised Weapon that the Longbow has becomes has none of the Longbow's original weapon properties (Ammunition, Heavy, or Two-Handed) by default, but the DM is invited to consider what properties it should have. 

Both of those are rather significant rulings, to not have any backup in the text of the PHB. The first I'm tempted to get behind as RAI, but the second I think has ramifications far beyond the off-the-cuff ruling he's making there. While it's reasonable enough to say that a DM can decide that a curtain rod is an improvised weapon (d4, no proficiency bonus) that deals blunt damage and has the Reach property... the Improvised Weapons section in Chapter 5 doesn't actually invite DMs to do that kind of off-the-cuff weapon building. It asks them to 1) decide whether it's close enough to another weapon to be "treated as" that weapon, or 2) if not, have it deal d4 damage, and have a Thrown range of 20/60 if you're throwing it. 

I think that JC is probably tapping into a more robust and detailed version of what the rule could have been, had it benefited from more playtesting and been given more room in the PHB. Honestly, Improvised Weapons needed to have more room to answer these kinds of questions, but suffice to say, none of what he is saying is actually contemplated by the RAW language that was printed, for whatever reason. 

I would argue that if the melee attack with a ranged weapon was not intended to be identified as an improvised weapon, that information would also appear outside of the very clearly named rule "Improvised Weapons"

I don't disagree that it could benefit from more fleshing out but I imagine the idea is that the rules set a principle (you can use improvised weapons, here are some examples, and guidelines for use) but the actual improvisation of the weapons amd their impacts comes on in play.

There's a few objects in adventures that have a little bit of description about how they would work as an improvised weapon that I feel agrees with this interpretation