Chicago aviation security officers are not supposed to board planes unless

[pnwmom]

9. There is no possible definition of imminent threat that applies to the doctor.

Every single report I've seen says that he was behaving perfectly well till he was illegally man-handled by these airport thugs.

Also, the Dean of Cornell University explains why UA shares legal culpability here:

http://www.newsweek.com/why-united-were-legally-wrong-deplane-dr-dao-583535

Although this depends on the facts, news reports suggest that Dao was not upset, and was minding his own business until he was told that he was being involuntarily removed and he was dragged kicking and screaming from the aircraft.

SNIP

In contrast, the object and purpose of the contract of carriage is, among other things, to require the airline to transport the passenger from location A to location B aboard aircraft C. Being on the aircraft is the whole point of the contract, and it specifically lists the situations when the airline may deny transport to a ticketed customer.

Since the airline did not comply with those requirements, it should be liable for the damages associated with their breach.