The downvote option does make it easier to be negative, but it also gives people an option in between ignoring trolls (and risking people thinking that other people don't have a problem with them) and engaging them (and rewarding them). It also lets people express disapproval of a post without wasting another post saying nothing but "I disagree". Of course, people still do downvote and then also post a comment that has no semantic content other than "I don't like you".

At a party, there are all sorts of feedback: smiles, laughs, nods, frowns, awkward silences, glares, etc. The upvote/downvote is a rough analog of that.