Well for legal reasons, many of them do claim to charge money for the greeting and chatting, and whatever two consenting adults happen to do after that is their own business.

It's true that they probably couldn't charge for the greet and chat if it weren't for what they do for "free." It's also true that Mike Perham probably couldn't charge for his commercial work, if not for what he did for free.

Admittedly this is a rather loose analogy.