Dr. David Dao, the victim of the incident, has retained legal counsel (including a very well-known personal injury lawyer in Chicago specializing in aviation disasters).  There's an interesting argument by a law professor here about which rules in the "Contract of Carriage" actually govern the situation, given that Dr. Dao was already on board.  If a court endorses this interpretation, I'm sure all airlines will start revising their contracts.

UPDATE:  Jens Ohlin, a law professor at Cornell, also weighs in on United's apparent breach of contract with Dr. Dao.