This isn't a comic book. Whether or not you think Anonymous are mostly good or mostly bad, there is a simple fact about them: they are humans. You are aware that people are not strictly good or evil right? Most hardened criminals still love their kids, and most regular Joe's have asshole moments or have even committed crimes! (think, half the people in the us are guilty of using illegal drugs at some point, half have driven drunk, and so on).

I'm sure if anonymous tracked down the stuff and told the police they would all have been arrested for child porn themselves, as this seems to be well on the side of "going to jail", based on precedent previously set by prosecutors all over the US. Further, (because details are lacking) if the server and/or the viewers are not in the same country, full prosecution becomes quite difficult.

But whatev, they did a bad because they are anonymous. No credit to them for stopping something that proper authorities were apparently ignoring or completely ignorant of -- either way those authorities were being completely ineffective.