The terms can be confusing at times, but they shouldn't be if used properly. Every "open mic" I've been to in a pub comes with PA and mic - and you would surely expect that from the term "open mic".

The "acoustic" sessions I go to are just that - no amplification whatsoever, and everyone who goes to them appears to know that.

Sounds like it's some pub landlords who are mis-using the words. Mind you, I've yet to suss out what some mean exactly by "unplugged" on certain T specials...