I personally do not understand how somebody can support United or the law enforcement officer. There is a contract between the airline and the client to transport the client from A to B. Client got booking confirmation and paid for the specific flight. Client checked-in. Client boarded the aircraft. All in agreement with the airline.
Then suddenly the airline changes its mind and enforces the breach of contract with a law enforcement officer.

Does not sound too “civilized” here.

What would you say to this example: I sell my house to you. We sign the contract. You pay. You move in. And the moment you are installed and a your furniture is placed in your new home, I enforce with some law enforcement officers access to your new home and let you throw out, because my son needs the house now.
Would you accept it? Would you say: Yeah that’s OK. I paid for the house. We got a contract between both parties that the house is now mine. But if your son need the house now, I am fine.