As to the situation you describe, I do know that there are open mic situations here in the U.S. that require performers to pay the cover charge just like anyone else.

There has been a long tradition of musicians playing for their supper (and sadly of having to) - and in my view it would be regretable if this aspect changed due only to being seen to be sticking to the letter of the law. As there are ways to be found to get around this without being slave to the rules - as my example demonstrates.

I feel sure that the places you describe would not stick to their rules to extent of denying their audience the opportunity to see some well-known performer who happened to turn-up to play - just because this performer may not agree to pay to perform or expect to.............They may even let their spouse and children in to watch too.

Hopefully, obtaining the best music possible for an audience would always be thought to be most important consideration?

There are such places as you describe elsewhere in the world but I would not attend these by paying to perform. In my example, it was I hope clear that the question of paying for admission was not specified in the advertising nor when I booked my spot. Had it been, as I had no money to pay for this - I would not have booked my spot or have been interested in performing on that basis.