These licenses follow copyright law, not contract law. If they were contracts, there would be little question of whether they were enforceable and "GPL enforceability tested.." would not appear in nearly as many headlines as it does.
That is what's so brilliant about the GPL: if you accept the contract, then you are bound by it's terms. If you reject the contract (or plan to argue in court that it for some reason is invalid or does not apply to you) then you are still bound by copyright law!
