One Spell Slot Per Turn RAW and Monster Casting?

In 5.5, the RAW on casting is "On a turn, you can expend only one spell slot to cast a spell. This rule means you can’t, for example, cast a spell with a spell slot using the Magic action and another one using a Bonus Action on the same turn."

Since monsters stat blocks with spellcasting, generally read "X/day" and don't say anything about expending a spell slot, does this mean that monsters can cast more than one leveled spell per turn? If that's true, that means the Archmage could Action: Fireball, Bonus Action: Misty Step, Reaction: Counterspell all on the same turn?

Is that right? 

Yes, that's right.

DnD is by nature an asymmetric game. Rules which constrain PC's do not necessarily constrain enemies controlled by the GM.

As long as the monster stat block does not mention spell slots, that is correct.

However, I believe the only official monster stat blocks with spell slots will be the 2014 ones that haven't been updated. I go ahead and run those as spell slots with that limitation.

Monsters also get lair actions, legendary actions, legendary resistances, and other features not avaikable to player characters. 

Kill a lich, its phylactory will bring it back in a few days.

Kill a fiend while its not on its home plane, it just reforms back home.

I agree with the opinions above. Here are some related threads with more (and interesting) opinions about the topic you asked about:

- 2024 monster spellcast vs counterspell
- Limitation on casting multiple spells per turn in 2024 
- Do Monster's "X per day" spells count as "Spell slots" 2024 
- Counterspell