Every time I read a post here asking "would this be illegal?" or "could I get arrested for this?", I think about George Carlin's old routine about sin and the Catholic Church: "eh Fodder, aaaah, so what if you're on a ship at sea... and you haven't received Communion yet, and it's Easter Sunday, and now it's midnight - too late! But den, you cross the International Dateline! Would dat, den, be a sin, den, Fodder?"
I don't really get the purpose of making up all these elaborate scenarios to ask "would dat, den be illegal?" In this country, it doesn't really matter whether it is legal or not. The real question is, can you be arrested? Can you be charged? Can someone cause you embarassment or make you spend money on a lawyer?
For instance, a lot has been made of the fact that prostitution is not expressly illegal in Rhode Island. But that doesn't change the fact that many people have been arrested in Rhode Island for engaging in prostitution, then turned loose because the DA could not prosecute. They still got their name in the paper; they still sat in a jail cell for a while - and some probably had to call their SO for a ride home 
When, and why do people get arrested for prostitution? Do you believe that there is an Eliot Ness wannabe out there researching the hobby and travelling from state to state, systematically taking out providers and hobbyists alike? I don't. And most LE doesn't really care about this hobby as long as it doesn't make trouble in other areas. If they are getting daily complaints about streetwalkers, they will react. If there is serious drug trade related, they will react. If a crime is committed in conjunction with the hobby, they will investigate. and so on. Hypothetically speaking, if I were the type of fellow to consort with providers 
I would be more interested in what the local LE climate was than how the specific laws were written - have I read about recent busts, has the mayor vowed to clean up the city, is it an election year? Then I'd research the provider in whom I had interest, be careful and follow the rules while booking, and in the end run keep my fingers crossed, since no matter how careful you are, shit happens (I read that on a t-shirt).
I don't know why I'm ranting about this. It's a slow day at work...