I always wondered about that actually. They could really prevent a lot of problems by not allowing it to begin with. Because just because one can easily get around it with just two accounts doesn't mean it wouldn't reduce the abuse (from people that are too lazy to do this for example).
I don't expect explanations to help very much since most people would simply write "disagree with rewards" or something. But who knows.
the idea that people will just go around it rotten indeed.
agreed that downvoting moderating would be difficult.