A contract, in the strictest sense, is not one sided. It is agreement between parties engaging in some kind of
mutually beneficial activity. All sides of the contract have obligations and requirements they have to live up to. You seem to be so willing to hold this guy to his end of the bargain, yet in reality, United is the one that put themselves in the condition to not hold up their end of the deal.
United, in your would of "one sided" contracts, can void the contract without penalty, while the customer can suffer significantly.