I think the mentality that downvote = disagreement is widespread on any site with a karma/voting mechanism. Many subreddits themselves say right in the sidebar (and often elsewhere) that "downvote only if something is off-topic or does not contribute to discussion", but everyone still tends to use it that way.

So I think the issue is somewhat bigger than reddit or HN or even just a certain slice of users. It seems to be a fundamental problem in understanding up/down votes. Maybe psychologically we're inclined to downvote when we disagree, whether consciously or not.

I am also guilty of this even though I know better. It's hard to resist downvoting something I disagree with. It's like a natural response I have to actively work against.

Edits: grammar / clarifying wording