Unless you've got something newer to suggest otherwise, that's how HN operates. "Downvotes aren't for disagreeing" is an absurd redditism (everyone knows that they are, since upvotes are for agreeing).
I reserve downvotes for rudeness and demonstrably wrong/misleading comments.
Personally, I use upvotes to reward new knowledge or an interesting argument in a comment, I don't upvote every comment I agree with, and downvote only if the comment is offensive.