Because you're using your power to override the positive opinion of someone else who gave their upvote. And the real reason you're giving the downvote is something else entirely: to combat Hive inflation according to your own criteria. I believe quality should be rewarded with upvotes, not punished with downvotes. Downvoting for plagiarism, spam, or other reasons is a valid reason. It's also valid to use it to lower a post's rating to reflect reality. It's simple: what's the point of downvoting for 0.35 cents, for example, someone who spent an hour creating a post, who put in the effort, even if you personally don't like it, if you then upvote someone for a comment they made in seconds for more money? By redirecting rewards, you end up voting twice: once with an upvote and again with a downvote.