Pre-emption would perhaps work if the GPL was trying to base itself on a state law, but it doesn't. The GPL is a way for an author to easily exercise the rights that USC-17-102 says the author has exclusive rights to grant. One of htem is the right to authorize reproduction ... and USC makes no mention of the terms of the granting - you could grant the right to a public performance of your play for no royalties under the condition that the performance be in the nude. It's legal.