From today's letter section of the Star Tribune.
Dao was firmly within his legal rights to refuse deboarding, according to United's own contract for carriage. While it is true that a passenger may be denied boarding due to overbooking (under Rule 25, Denial of Boarding), once a passenger has been boarded, the rules change. At that point, Rule 21, Refusal of Transport, is the applicable rule; it specifies why a boarded passenger can be thrown off a plane, and neither overbooking nor the convenience of the airline is on the list. In fact, no passenger then boarded on Flight 3411 met the criteria of Rule 21. So if United was unable to bribe passengers to give up their seats, it was contractually obligated to fly with the people on board, and its crew would have had to find another way to get to Louisville.
Not only was United morally wrong, it was legally wrong, too. And it appears that no United employee, including the CEO, has been properly instructed in contractual obligations. Fortunately, it seems that Chicago's airport police have realized that they are not hired muscle for the airline; all three officers involved have been suspended.
Keith Pickering, Watertown