It is a police protocol. I listened to a police interview today and the officer said that if a suspect is resisting to put their arms behind their back (like in the video - the guy just lays there with arms by his side and doesn’t put them behind his back) this, as per protocol, threatens a police officer as the suspect can easily reach for the officer’s gun.
You can only question if it is acceptable use of power if you have familiarised yourself with protocols and, most importantly, why these protocols exist.
another protocol is to shoot anyone on the spot if your fellow police officer’s weapon has been taken by non authorised personnel. And yes, these protocols keep us all safe. Questioning these protocols makes us all vulnerable.