Simple. Someone shares their opinion about a topic and is making it's clear it's just an opinion, not an answer. (Then again, opinions are a kind of answer too.) Now, would it be okay to downvote opinions when I disagree with them or not? In my opinion, downvoting should be done to show the bad answers. But an opinion can't be bad, since it's just an opinion. Just like an opinion can't be good and thus we shouldn't upvote either.
This is just my opinion, btw. If you agree or disagree, leace a comment. Don't vote for this one! ;-)