I'm totally shocked by this thread. Weren't you the guy like not that long ago, JandK, who kept arguing that Larian is awesome for not having monsters with proper 5e stats and abilities? Now... you're saying Larian shouldn't have done such things?
I'm so confused.
It's not confusing.
There are rules for players. I think the NPCs should, by and large, follow those same rules.
That doesn't mean those rules should be exactly congruent with 5e in every circumstance. That's another conversation.
The point is, whatever the rule is for the PC, the NPC should roughly have to abide by it.