Voting for a witness is like voting in an election - you downvote all those you don't vote for. You don't have to use all your votes, the less we use the more worth are the votes we did use.
"Downvoting" on Steemit isn't a negative vote per se. It uses voting power but it does different things.
Similar to a real election, you only vote for a person or a party. But if none of the candidate are worthy, then you don't vote any.
Many people are always saying "Don't waste your witness vote! You can vote for 30 witness", and I detest it. Why do I have to force myself to spend all 30 of my witness votes? I will only vote for witnesses whom I believe are contributing to the blockchain or community. By voting for them and not voting for the rest, I am essentially downvoting the unworthy ones.
The other responses to this are targeted to 1 vote = 1 vote. Currently in our system we have one account whose vote is worth what the majority of the rest of our votes combined. When they vote they literally decide who will be in the top 20 witnesses. A down vote would show lack of confidence in a witness. There is no current way to show you don't support a top 20 witness.