Turn it around and it's just a matter of not allowing anyone else to infringe on your rights to that extent; to go back to his example maybe you do have the right to sign yourself away to slavery, but nobody has the right to take up that contract.
Everyone uses the "signing yourself into slavery" example, but it's not clear what that means. If you consent to it, then it's not slavery (you can make arguments about "wage slavery," but I think that's a different issue). If you sign a contract to perform labor for some period of time, then change your mind later, the contract likely has some terms for breaking the contract early, and if it doesn't, it's probably not likely to be enforced by any court.
