I don't think that it matters that this is a "cheating" site, your privacy should still be guarded and it's not fair that all these people are being exposed.
Secondly, I'd wager that many of the users sign up to use Ashley Madison for the thrill of the idea and probably aren't actually cheaters themselves or hooking up with other cheaters. A lot of the users are probably into the idea of roll playing or something similar.
It's just too bad because I think due to the nature of the site a lot of people's private information is going to be leaked because this hacking group thinks they are doing something righteous and justified.