ok, nobody ever said you couldnt vote zero. but the point is so you vote fairly. voting zero is only for stolen movies, or movies with something thrown together for the sake of submitting for the hell of it.
the problem is, the zeros are being used too loosely. for example:
someone might make a well done movie saying drugs are bad. so someone comes along and watches it and thinks "man this is wrong, i smoke weed all the time and it isnt like this" so they vote a zero.
one thing that is missing alot of times is constructive criticism, and thats a large portion of the problem.