I suggest a slightly different view of that "cannot." Taken literally, the statement becomes "it is not possible for UAL to breach the contract."
What are the consequences if UAL breaches the contract? What if UAL removes a (potential) passenger without cause? UAL guaranteed a seat on a specified flight, and failed to deliver. It may have also committed the faux pas of allowing the passenger to board, which creates an academic discussion of the precise mechanism whereby the guarantee (and maybe contract) was breached.
Airline Captains are gods and passengers will submit to whatever god decrees. Submit or be beaten.
Simple.