Well if the contract and the law says they can involuntarily remove passengers from the plane, then it is necessarily legally permissable for United to assault/

Well if the contract and the law says they can involuntarily remove passengers from the plane, then it is necessarily legally permissable for United to assault/battery their passengers. Whether or not that is morally the right thing to do, or whether that is a wise business move in this case is a separate question (probably answer is no). Hypothetically they could legally shoot their customers if battery was not enough force if the contract stipulates so.