The theory is that you start the contract with the terms specifying that changes put forward by the company (but not the user) are automatically accepted with 30 days' notice. That's where the meeting of the minds occurs: in theory, from that point on, you've agreed that the terms can change.

However, I'm not sure if that's ever been tested in court as a valid theory, and regardless it certainly shouldn't be legal (any more than noncompetes).