You're right, it isn't hard to think up situations where passengers would have to be deboarded such as weather change, another plane crashing into them etc as you noted. However all of those items you mentioned are noted in the United Contract of Carriage which mentions force majure including weather, and damage to the airplane, running out of fuel, strikes etc. However, none of those reasons applied to Dr. Dao, and his being boot from the plane which why he would have a very good chance of getting far more than a refund. You can also be sure, eventually United and other airlines will quietly update there contracts to allow them to legally boot a customer under any circumstances at their sole discretion.