I think many people default to downvote-when-disagree. It's partially the "like/dislike" culture, but I think it's mostly because it's the opposite of upvote-when-agreeing.

I agree with you that downvotes are useless when one simply disagrees and don't help the author or the reader learn. With the "flag" feature though, I am not sure I see the point of downvoting at all (other than when someone repeats the same disproven argument in the thread): upvoting is there to indicate "me too", but any such thing turns into a popularity context.