1. It's possible, and even highly likely, that the person on the site never had an affair. Ashley Madison has been sued before for fake female profiles, and some are reporting as high as 95% of profiles being male. So I don't think it was that efficient at it's purpose.
2. There is a big difference between cheating and public humiliation. I am not saying cheating is okay, far from it. But the punishment here can destroy a life, a reputation, for far more than the relationship. Of course the cheater is not okay (in current society's moral compass, anyway), and the relationship is important to the SO, but it could lead to someone losing their job (think public persona) to their career and pension (think military). Being a scumbag isn't cool, but it's hardly worth that I'm sure.
3. You have to separate the two. Posting private informations on the public web should NEVER be a good thing, whatever you think of the people being doxed. THAT is illegal, and can definitely destroy lives.