[ISO-morphism]

In the US, copyright infects every work at conception. No one but the author has much of a right to do anything with a work without a license. The GPL is a license - it's a subset of big C Copyright. Without Copyright the GPL is unenforceable. The GPL can't infect anything that isn't infected with the notion of Copyright.

Every license, other than completely opting out of copyright and into the public domain, infects the derived work with some sort of terms. You'll see in a lot of proprietary licenses "No reverse engineering, etc." The GPL forbids you from forbidding anything except forbidding.

EDIT: nuance, "much of a right to do anything" should more specifically be "anything involving distributing to other people." There exist exemptions in copyright law for using copyrighted works for your own personal use. Giving that thing to other people is no longer personal use.