It turns out that the contract has a specific rule regarding “Refusal of Transport” (Rule 21), which lays out the conditions under which a passenger can be removed and refused transport on the aircraft. This includes situations where passengers act in a “disorderly, offensive, abusive, or violent” manner, refuse to comply with the smoking policy, are barefoot or “not properly clothed,” as well as many other situations.

There is absolutely no provision for deplaning a seated passenger because the flight is oversold.

An added complication here is that the flight wasn’t even oversold. The contract defines an oversold flight as “a flight where there are more Passengers holding valid confirmed Tickets that check-in for the flight within the prescribed check-in time than there are available seats.”

In this case, the airline attempted to remove seated passengers to make room for airline staff requiring transport to another airport, not because it had sold more tickets than there were seats available.

It's not looking good for United and the operator of the flight, but if he wins, blue skies for Dr. David Dao. (Pun intended)

My opinion: He was mistreated by all counts. Period. At the same time, listening to authority is what I would have done only because I wouldn't have know my rights in the difference of boarding and oversold.