Because down voting is a way of saying you disagree with something instead of posting, that's the point of it. And if you can upvote, and praise/agree with someone, but can't do the opposite, that's a bit of a hobson's choice isn't it?
I should probably shut up before this gets into a free will/censorship for 'good' discussion, but the point's there, why have one without the other, it's genuinely a stupid thing. And i can't tell you how many other forums get along really well with plenty of 'dumb' ratings. Facepunch has five bad ratings, one of which is for when someone insults someone else, and you agree. And it seems to be doing fine, and has done for seven years now; and continues to be one of the best forums for modelers and modders there is