The professionals almost always refuse to have sex with their clients. They just act out BDSM fantasies. I've always wondered if it was still considered prostitution even though there is no actual sex taking place.
I do know a professional body building Dominatrix and she makes a great living off of it..... she's highly advertised and is a US citizen so.... I don't think it would be any different then hiring a stripper. (I mean, yes it's different but... the fact that you're getting entertainment, sexual enjoyment WITHOUT the sex).
The difference between what's legal and what's not really comes down to... who pays taxes.