You should go back to law school. They can eject anyone at any time for any reason.
http://www.express.co.uk/news/world/757712/passengers-rejoic...
http://www.detroitnews.com/story/news/local/wayne-county/201...
Both of your examples would violate Rule 21, H, 1, and therefore removal is allowed.
In the case under discussion, the passenger was not in breach of rule 21 for any of the listed reasons.
This whole act of struggling with the police is never going to get a good outcome. It will only escalate things from a civil matter to a criminal one.
United asserts, they prioritise their employees gaining seats on the flight because their employees are actually necessary to the operation of this flight and others.
It makes business sense, and its logical.
There's no law on any book that will force someone to serve you, whether it be transportation wise or by letting you have use of their mobile property (land/residences are a special case). United can pick random people at any time to get off the flight, for any reason, and at worst it will be a contractural breach or a violation of a civil statue (like the overbooking clause).
Once he was told to get off, he should have gotten off.
It was United, not the passenger, who escalated to violence in a purely civil matter. He did not force himself onto the plane, he simply declined to give up the seat he had already been given.
CFR 250.2a covers overbooking but only applies before boarding. Since the United employees had no confirmed reserved seats on the aircraft, they cannot be given priority over a customer with a confirmed reserved seat.
The passenger was already boarded, so they cannot rely on oversales rules to refuse service. Instead they need to rely on their Contract of Carriage which does not allow them to eject customers for any reason whatsoever. The most circular thing they could eject him for was failure to follow the terms of the contract of carriage for refusing to disembark. They publicly stated that their reason for asking him to leave was because he was selected by a computer, but this is not allowed by their rule 21.
They had no legal right to refuse him service at that point.