Part of the ethos of free speech is that those who think your comment is absurd can say so. On HN, the downvote button is one way that people say so.

Now, it's a pretty poor way to do so, because it conveys only one bit of information. Still, it is within the site guidelines that "downvote to disagree" is legitimate.

Yes, my criticism of the down vote is as much a criticism of how HN has decided to structure things as it is of those who actually do the downvoting.