In case it's helpful: people sometimes think that we enforce this standard because we're uncomfortable with intensity and only want (say) a nice and civil bourgeois discussion. And they make legitimate objections to constraining discussion for this reason. But it isn't our reason at all. Rather, it's because of the systemic effects such comments have on a large public forum like HN. Experience teaches that they lead rapidly to lower-quality threads, in which people drop thoughtfulness, start to yell, and eventually just try to blast and hurt each other. Since the whole point of HN is to try to sustain a higher quality level on the internet, we have to moderate such comments rigorously.
So when we moderate HN, we're not thinking so much of the specific comment we're replying to, but what that type of comment leads to. I don't usually explain this because, as you can see from this post here, it takes too long.