That's a private matter between the people directly involved. Maybe their partner would also be humiliated by the exposure of their private affairs (I know I would), in which case it makes things even worse.
If you were only telling the person being cheated on, you might have a point. But since you're now telling the entire world that someone was cheated on, or maybe not even cheated on, you've kinda lost that.
Well the point is that the hackers picked a pretty good target. You're never going to convince me and millions of others that exposing cheaters is a bad thing, even to the entire Internet.