It must be stressed - repeatedly - that this was not an overbooking issue. Overbooking procedures are completely irrelevant to this case.
It is a far more important issue to face. Once passengers have boarded with a valid ticket and are not a hazard (to be defined in great detail) to the safe operation of the aircraft, does the Airline have any right to eject them from the flight for their administrative convenience.
In this case to make the next day's operations more efficient, UAL decided to eject 4 peaceful boarded passengers and replace them with 4 non-rev DH crew who had not bothered to reserve seats and had not bothered to arrive before boarding commenced.
THEN
Given that the passenger who refused to deboard was completely within his rights according to UAL's conditions of carriage and FAA CFR 250 under what rule were UAL operating to call 'law enforcement' and what law gave 'law enforcement' the authority to cause grievous bodily harm (it was luck that the passenger was not killed) to a passenger who was acting within his rights?
I repeat that this is nothing to do with overbooking - that is a pivot being made to avoid the far more egregious physical assault on a passenger for UAL's administrative convenience and to recover from errors made by UAL Ops and by the DH non-revs.
If anyone is to make any rules they should be to make it absolutely and completely plain to the passengers, the gate agents and the flight crews (including the more authoritarian commenters on here who think everything was done correctly) what the law is and what the rights of a passenger are. Including right of redress that I would suggest is more than the top limit that Delta is offering when the airline decides to void its side of the contract of carriage. This should include similar amounts to pax bumped by the 'clever scheme' of faked mechanical problems then bumping them on reboarding.
If the airlines do not sort out bumping to carry non-rev DH crews, then I expect DoT or Congressional decisions that will be indeterminate and likely to be far more damaging to the airlines. As causing grievous bodily harm to a 69 year old man has removed all sympathy or empathy with the airlines. The more 'coastal' comments made on here the less sympathy there is for the airlines' case. Remember that it is UAL that was caught out now but the lack of sympathy will be across all airlines - your jobs may become far more difficult and I would suggest everyone stops digging because climbing out of this hole is going to be painful for the industry.