Its their plane, their rules, etc. Deal with it folks.
^^This is how a serf 'thinks'.
If you want to know how airlines get away with this over and over, that's how.^^
Airports are built with public money. They are maintained and guarded by public employees. Most of them lose money and are propped up with tax revenue. Airlines buy their equipment from an industry heavily subsidized while it piggybacks off military research. The sticker price of a 737 would be astronomically higher if Boeing only made civilian aircraft. And after all these advantages when an airline finds a way to lose money (after decades of terrible service) the public bails them out.
They use our airports, our TSA, our air, our roads and ramps, and our emergency workers when one of their pilots parks one in subdivision. The least they could do, the absolute minimum, would be to not beat their customers unconscious while cancelling a service that has already been paid for.
Make them auction the seat on the spot. And we get to know if the seat is going to a dude trying to get home to a delivering wife or because United needs to squeeze another shift from a flight crew. It would only take one committed group of passengers agreeing to hold out and split a 5 figure number to end this once and for all.