Doubt we know the whole story of what happened. Of course the evil big corporation is going to get blamed, but did he have any responsibility?
United should have stopped him at the gate. They should have said they were four seats short, and that Dao would be compensated and “re-accomodated,” to the best of their ability.
What could Dao have done? He wasn’t going to drop to the floor and start banging his head till he was bloody. United would have been in the clear.