Voting a post up is positive. Voting a post down is lazy. If you have a better answer, post it. If you don't, you have no business berating somebody else's insight and effort. This is not a American Idol forum, this is a technical forum. Hans Passant.
How are they any different? If I have the right to up vote an answer, I should have the right to down vote, simply because the answer could be wrong or off-topic. Either have both or none.