That hasn't been my experience. People show their dissent of opinions with downvotes here on HN too on sufficiently polarizing threads. I think the main difference is the type of content posted on HN vs reddit and the type of people who readily consume that content.

For example, I have noticed shameless downvoting in operating system war threads, both on reddit and on HN. In highly technical threads about physics or mathematics I have noticed people tend to vote based on the quality of the content posted.

Also, you need a lot of rep to get the right to use the downvote button, which also removes a large chunk of the would-be-downvoters