Except some Graal Players cause the problem on their own about complaining about how its copywrited and shouldn't be allowed. That isn't the real reason for it either, but either way I can hardly see players being allowed to upload customs that are obviously ripped from something else (I myself don't care, if you submit hats that is from some show or something, I really couldnt care less as long as it looks good).

Best solution however is to use some creativity.