As a long-time HN user I completely agree with you, but it's worth mentioning that pg specifically stated that it's perfectly fine to use downvotes for disagreement (if I find the comment where he said that I'll edit my post and put a link to it). So, while I don't think that downvotes should be used for disagreement, if the person that created this site says that it's ok, then I guess it's ok.

EDIT: Ah, here it is: https://news.ycombinator.com/item?id=117171