It's not the same as "violating someone's privacy" because that implies that these people shared something deliberately with someone they trusted, that wasn't the case, this was stupid people posting shitty, hateful, vindictive or exploitative material in a semi-open space and not being careful about who they invite in. Fuck them, they deserve this lesson.
True, but it is oh so satisfying when bad things happen to shitty people. Especially people finding out the true depth of their shitty. Even more satisfying when they complain about it.
I think it's pretty comparable. The guys in the OP are getting mad at Derek but they're the ones who said bad things in the first place. Their actions caused this. All Derek did was let people know the truth, and maybe it's good when people know the truth.
In both cases you have a person doing something that many people would consider bad. Then this person gets mad when others find out about their behaviour. If you don't want people to find out you did something, maybe just don't do it in the first place.