I'm sure it's just something like an averaging system where each vote counts from 0 to 4, or some such. So just vote the worst you can if you feel negatively. It's just a matter of perspective anyway.
Well, negative or positive, could we at least make the meaning of the little stars a bit clearer? Maybe empty stars could actually be a completely empty graphic so that only positive stars are visible? The coloration method has me foncused.