To be clear, it was not United that forcibly removed the passenger, it was Chicago Airport Policy/Aviation Security.
The passenger was forcibly removed for non-compliance with Police instructions. The Police were called after he was non-compliant with instructions from United Personnel and Crew.
Meaning, the passenger could have walked off untouched after being asked by either United or Airport Police. Once he refused to follow Police instructions, he was removed by whatever means they, Airport Police, determined was warranted.
Once Law Enforcement were on scene, especially after he refused to comply, the situation was essentially out of United's hands. They could not have prevented the removal even if they wanted to.
Outside the specific scenario of an airplane and video, this is an otherwise routine Law Enforcement action.
United was well within the law and contract to remove him from the manifest of the flight. Once that is done, he cannot remain on the aircraft.
Use of Force is widely recognized in Law Enforcement. This situation would likely be governed by the laws of the State of Illinois: PEACE OFFICER’S USE OF FORCE IN ILLINOIS