Contractual discounts--which corporations do have--are usually balanced by a guaranteed spend figure. For small and midsized companies, the spend is often paid in advance, to the benefit of the airline, which can carry the float and lock up a portion of market certainty.
My two cents: I have a deep amount of respect for United's tech stack. It is the best amongst US airlines and among the best in the world. However, this is the wrong move for them, and unless other airlines follow (doubtful, since United taking the lead here gives others competitive advantage,) United will have to backtrack in a year or two.