It's simply the practical question of what type of site HN is trying to be. We want curious conversation, not people bashing each other, which is what flamebait leads to.
Commenters who get too sure of their moral positions tend to do a lot of this. I'm not saying the moral positions are wrong (I probably agree a lot of the time, certainly Sinead did a courageous thing, etc. etc.). But the quality of discussion this leads to is predictably poor, evokes worse from others, and tends to go straight to the bottom of the internet barrel. We're trying to not get sucked completely into that muck here—a difficult task to even partly achieve, so we need everyone's help.