SO after reviewing a number of threads, where DM's complain about RaW limitations, confusions, and so forth, I asked myself "What kind of DM allows a silly, or poorly worded rule derail the fun and enjoyment of their campaign?"  My only available answer was: "Ones who play with strangers and a myriad of different groups all the time."

I am wondering how many DM's fret over minor details that get in the way of their story and how many chuckle, house rule and move on.  Now I'm not talking about core, hard-lined rules, but more of fringe stuff and most specifically, enemy behaviors/abilities.  Most recent was how enemy spellcasters are presented to us and what their little sheet says.  Now, to ME, an enemy that is a caster, has as many spells and slots (as it were) available as I think they need.  They have prepped any protections or buffs that would be appropriate, used any spells they would reasonably have before the party arrived (scrying, maybe an alarm spell across the entry passage, etc) and generally, handled themselves as a Pc might in that situation.

Example given there was a Drow Priestess.  She would, upon knowing hostiles were coming, cast protections on herself, maybe invis some of her minions, and so forth.  It isn't on her stat block, BUT it is perfectly reasonable to presume she would have an alarm type effect on the entry to her home, that she would have proper combat spells and that she would take steps to properly defend herself and her home.  I refuse to be limited by a poorly worked up stat block and I will, as required, make adjustments on the fly, to keep my enemies in line with what makes sense for them, regarding their race, class and station.

Of course, this is easy for me, who plays with a steady group, so we don't have the dreaded rules lawyer around.

This doesn't even have anything to do with RAW. 

RAW clearly says that it's ok to do what you are doing. From the DMG:

"Part of the D&D experience is the simple joy of creating new monsters and customizing existing ones, if for no other reason than to surprise and delight your players with something they’ve never faced before."

The point of a rule system is to make the DMs life easier by meaning they don't have to house rule everything. Rules that are incomprehensible or broken are failing at the purpose of rules.

I think a lot of it comes down to being in a long term group. I trust my DM that if he makes a ruling or has something contrary to the rules, that it will all be ok. I know he’s going to be fair, and he has a good reason, and it will work out.

And also, a DM trusting in themselves. Particularly if they are new and running a published adventure, they’re afraid they’ll break the game if they do something that not written down. 

Not only it's within the rules for DMs to customize monsters and NPCs, there's even guidelines for modifying creatures in the Monster Manual;

Modifying Creatures: Despite the versatile collection of monsters in this book, you might be at a loss when it comes to finding the perfect creature for part of an adventure. Feel free to tweak an existing creature to make it into something more useful for you, perhaps by borrowing a trait or two from a different monster or by using a variant or template, such as the ones in this book. Keep in mind that modifying a monster, including when you apply a template to it, might change its challenge rating. For advice on how to customize creatures and calculate their challenge ratings, see the Dungeon Master's Guide.

True, monster stat blocks are part of it, but too, there are spells and such that could (should?) be allowed to do things "not" specifically listed.  Fire bolt doing significant damage to an ice wall or area of snow.  Gust moving a levitating character a few feet.  Spell effect doing things that make sense, even if the spell description doesn't outline it.  Nothing game-altering, but stuff that adds to the fun, allows more creativity and opens the doors to imagination, as opposed to being rigidly stuck with specific wording.

It's also part of the DM's job to prevent or allow things "not" specifically written in any rules, spells, feats, actions, traits, features etc. . Beyond RAW, a DM can make rules call,  adjudicate effects and allow things to happen in his or her campaign that it feels makes sense or otherwise improve fun.

I personally make a distinction between RAW analysis vs DMing, or Rules as Written vs Rules as DMed. The former is static as is and stands on its own while the latter is flexible, processed while influenced by human factor, affected by a lot of factors of consideration, situation, context etc... It's interpretation powered by imagination, inspiration, improvisation etc that what's making roleplaying game so unique!

Rulings, not Rules.

DMs run games by adjudicating the actions of the PCs and the outcome of the dice, the rules have yet to run a single game.