I'm not totally clear how this is illegal, unless these guys didn't get the right legal paperwork before they did the stuff. People fight in movies all the time. People fake fight with each other in the street all the time. It seems like one of the bums would have to press charges to make the police case work.
If they had the bums sign a contract/release saying they were acting in a video, then it seems like there's nothing the cops could do. If they didn't get all the right paperwork, and they were paying these guys to commit crimes (assault each other), then watch the bums turn around and sue everybody involved.
Roofus the Stunt Bum might become Roofus the Rich Motherfucker if he gets the right lawyer!
Personally, I think the Bum Fights guys should be able to get out of this if they get the right lawyers and make sure the bums don't turn on them. It's just acting, right? Wink, wink.
Where is Law Pal when we need him?