Police have a terrible job, as
@Shit Plate said, so they overdo it on a lot of things because of the worry for their own lives that comes with being on the street. But why should their worry about their lives outweigh anyone else's chances of living? There are four or five officers on scene, plus the initial security guard who was there. The guy was cuffed on arrival and they put him down and added additional leg restraints. Had he gotten up, the best he could've done was try to hop away. Had he somehow had PCP strength and broken his cuffs, you would think between the four officers they would have had some protocol to deal with it. And that's where it comes down to training and protocols, as
@Splinty said. I've done enough BJJ to know that a knee on belly or back can put a lot of pressure down on someone. The cop alleviates some of that pressure with his hands on the victim's shoulders, but mostly in a way that redistributes it to keep the spine aligned and the face in the dirt. It's hard to imagine that this was the best procedure under any circumstances, but it's symptomatic of the poor training, wide breadth of discretion, and general "well, it's me or them" mentality that often leads to fatal encounters with police. Cops really need to be better, the best even, but right now they're far from it.