In addition, as I and others have repeatedly pointed out, once a suspect is handcuffed and restrained, the arresting person has custody of the suspect and is responsible for their wellbeing. You can not restrain someone and then shoot them, which is essentially what happened here.
They did not arrest him, they subdued him.They were acting as agents of their employer. I they reasonably thought they needed to sit on him untill the cops got there to keep him from getting up and fighting or running they had every right.