I think people are under the impression that pointing out how a popular band sucks makes them knowledgable on what is actually good in metal.
I agree, if I have no interest I don't comment. When I do comment negatively on a band, it's because it's a band I should be interested in but I think they've done whatever it is wrong/badly. So I do have interest, I just don't think it turned out well. That's called debate/discussion and has ever right to go on.
If I weigh into a thread about a death metal band saying they suck, because I don't like death metal bands. That's different.