Aren't there some limits on which legal rights can be signed away through a contract?

I imagine that a contract in which someone agrees to become a slave would be void.

Sure there are limits, but they seem to apply to really egregious things like selling your life or vote. We're talking about signing away the right to fair use, not entering into indentured servitude. People sign non-disparagement agreements all the time and this seems more like that than slavery.