There is a very fine line between saying "I will publish if you don't pay" and "I will keep it online unless you pay". It's pretty clear that this doesn't violate the federal extortion statute, but these cases have not been tested against laws in all 50 states. I'd say the odds are pretty good that there is at least one state extortion statute written broadly enough to cover this type of behavior. All it takes is 1 prosecution in 1 state where there are victims to start taking big bites out of these sites.