UA have acted disgraceful in this incident.
The passenger had paid for his seat for this flight. That OPS had to accommodate DH crew, was not the fault of the passengers.
Ultimately it is the Captain who is responsible to call the Airport Police to the aircraft. The Captain is the one responsible for all actions taken on his aircraft.
And I would expect the Captain to at least be called in for tea and biscuits.
Anybody who can condone the behaviour of the police, and how and more importantly why he was removed, need their heads examined. You clearly do not live and operate in a customer orientated world, or you clearly don't care about how your customers are treated.
The lawyers will have a field day with UA, and they will unfortunately probably reach a settlement outside court.
The passenger has paid for the seat for this specific flight, imagine you have a hotel room for one night, you have paid for this room for this specific night. So you have gone into the hotel room and gone to sleep in the bed, than shortly after the police enters your room and removes you for no other reason than to accommodate a staff they have working there.
This would not be right, neither legally or morally.
You have paid the money for the seat, and as long as you have not breached any of your terms and conditions, you have the right to quiet enjoyment of what you have paid for.
Any Captain taking this decision, should in my opinion be seriously reprimanded for allowing this situation to happen, including OPS management who allowed this to happen.
You have damaged the reputation of your employer, will cost them millions in compensation and bad
PR. These are one of those Non-Tech situations, that you do not get trained for in the Sim, where you need to show why you deserve to earn a 6 figure salary.