You can think of copyright as a certain kind of social contract between the creator of a work and society at large.
Copyright doesn't view an author's control over his/her work as absolute. In the US at least, Copyright is codified in the Constitution as a necessary evil - something that Congress is entitled to grant authors in order to promote science and the useful arts.
Copyright isn't all-encompassing. It expires. Some things aren't copyrightable. Other societal rights trump your rights as a copyright holder. You can't stop someone from writing a news report about your book, or writing a scholarly essay about it. Until the DMCA, you couldn't stop people from making a backup copy of a work. Even with the DMCA in place, you still might not be able to stop them. In short, there are several "fair use rights" that the copyright holder simply can't deny you, no matter how much he or she wishes to. It's simply not in society's interests for them to do so.
The GPL doesn't interfere with any of these fair use rights, in fact it encourages something much stronger than fair use. It says, "Please, embrace and extend this work." With traditional licenses, only a narrow amount of "embracing and extending" is permitted under fair use rights. The GPL serves to limit your rights as an author, where traditional licenses generally seek to maximize them. You're always free to give up your own rights, but you may not force others to give up theirs. So your analogy with the GPL's enforcability is a poor one.
Certainly under today's laws, authors of a work have the right to license their work under any contract they see fit. But then I don't think (from a moral standpoint) they're entitled to the protections Copyright would otherwise provide, because they've encumbered reasonable societal benefits and rights. Copyright holders simply aren't entitled to do whatever they see fit in order to "protect" their works, because the laws are set up to protect society.