Wait a minute. If this was "a violation of updated policy", then either he failed to follow it, or one of his superiors failed to advise him of it. Either way, the BLM mobs just may have a valid point for once.
Whether or not it is safe for the officer to reach around a prisoner to get the seat belt is a judgment call. There was testimony that there was a crowd gathering and it was getting ugly, so it is not unreasonable to skip the belt in the interest of getting the door shut and the officers away from the scene as quickly as possible.
Policies are not criminal statutes.
Laws are laws. Policies are simply policies. The City of Baltimore can discipline him as a police officer for not following a policy, but they can’t put him in jail for it.