They are not supposed to.

I remember a few years ago when a casino had a bank of $1 machines. One machine had a sign with 99% payback on it. The rest in the bank were higher hold. Someone, I assume gaming made them either change the sign or change the hold on all machines in the bank.

GCB does monitor advertising and the truth in it. Yes, they will step in if needed.
All Nv casinos release "Win" every 3 months and they are public numbers. It is very easy to tell who holds what, if you read local papers.