Yeah, I wondered. So within 30 seconds of digging, I found out why.
Anonymous appears to have got it wrong.
While the idea of faceless masses of vigilantes achieving what the police etc cannot is truly appealing (and I have personally cheered a couple of their campaigns on), it should be remembered that Anonymous "is legion" -- ie, comprised of a really large and eclectic bunch of people on the internet, the majority of whom do not actually know each other from soap.
And just as it is in any crowd made of loosely connected hordes of people, some are a whole lot smarter than others.
This said, outing a predator isn't a bad thing to do. If he really is one. They got that bit right, anyway.