I'm thinking of infringers in China, for example. You wont win any damages from them, but if the GPL says that by using it, you agree to also release you own code under GPL, then at least everyone could start copying China's code. And if they complain, just point to the GPL code they used and say "you agreed to this by using the GPL code in the first place".
That's not how the law works; you have certain rights, but you don't get to make up your own unilateral contract that says what you can do to someone infringing your rights, just like you can't e.g. steal someone's goat because they stole your cow (or vice versa).
That only works if they "agreed" to the license in the first place. Without a license, it's just copyright infringement which is controlled by statute. There is no shrink wrap license on source code.
