You're explicitly wrong about downvoting being a mark of disagreement. That's been black-letter HN culture since forever. I don't entirely like it either (though I'll admit that I do it, too), but 'pg repeatedly went on record, saying he meant for it to be that way.

That said, I can't stand it when people downvote comments pointing out objective, verifiable facts that they don't like. It happens all the time, and I despair a bit for the quality of discourse even possible in a community where that's a thing that people do.