Is it legal.
Is it moral.
Should it be legal.
The last is quite complex and I won't get to it here because of the vast differences in moral theories as to what should and should not be legal.
As to the first, is it legal? Hacking isn't. The end.
But the middle question is interesting, and I dare say that many people have views such as my own, where morality is not dependent on legality but on morality. And I would also dare to say that most people do not have an issue exposing people who are using websites to commit very immoral actions. So the question becomes one of if the action of cheating is immoral enough to cross this threshold. That question is a really deep moral one and you probably have as much ability to change as most any deeply held moral view.
In short, most everyone agrees the right to privacy has limits based on extremely immoral actions. The difference is only in what counts as immoral enough, which is far less a distinction than 'right to privacy means hacking is wrong'.