That seems like a cop-out avoiding the issue. If you did see it, what's the argument for it to be unenforceable? Note that it should be unenforceable, but it's a hard case to make without a certain amount of "I know it when I see it".
And "there are no limits to the content of contracts people freely enter" is obviously an untenable position – we'd all agree that a provision giving me your kidneys, hidden somewhere deep in the ToS is unenforceable.