If United PAID enough . . . . to buy back a seat, then many passengers would have scrambled to sell their seats.
But for the industry's shield of Fed Aviation Admin regulation, the airlines wouldn't even try this.
Under the FAA, airline passengers do not have any right to sue, except AFTER consideration by the FAA.
The rules that allow this kind of abuse are written by the FAA, and the FAA is a creation of politicians.
The FAA, as with all regulatory bodies, is subject to capture by the supposedly regulated industry.
The problem here is that United did not want to pay enough for the seat.
And that unwillingness is the result of regulation that does more to protect the industry than it does the public.
Generally, the public is better off with juries than with regulators. Abolish the regulators, let juries decide.
Fear of juries and judgments incentivizes good behavior more than does buying politicians and seats on regulatory boards.