Honestly, I think it's the "traditional" fanfic sites that wouldn't have a legal leg to stand on. Fanfic is infringement of copyright, and those disclaimers people put on fic would never hold up in court; most rights holders just don't care because it's not costing them anything. If they get it into their heads that they should be paid for fanfic, and that anyone offering it for free is eating into their profits... They'd be idiots, but they'd hardly be the first idiots in the media business.

I don't think it's anything that's likely to come up immediately, but once Kindle Worlds becomes an established source of revenue, who knows?