A contract by its very nature is a set of terms by which two parties agree to exchange things of value. If one of those parties can change any provision of the contract at will, with no possibility for negotiation and no additional value provided in exchange for being able to change the contract terms, then it's not really a contract at all.
The problem here is that AT&T has all of the cards. It can force me to abide by the terms of the contract no matter how onerous, can prevent me from accessing the courts when I have a grievance, and then has the power to change that very same contract that you and I have to abide by no matter what, on a whim. If that seems fair to you, then you must be an AT&T shareholder.