In addition, mate, their own rules states that the knee on the neck of a passively resisting citizen is strictly prohibited.
As Zin said, the US system is all wrong; when a cop has the mindset of 'better to be judged by twelve than carried out by six' before they leave for their watch, then it sets them up for the power of judge and jury/ siege mentality surge they have in their job role, day in, day out.
Whilst this is not the same for all cops, when they are working in high density cities with a high ethnic/ poor population, this mindset is clearly acute.
We see it all the time...
Small examples of a fucked up system.
"Protect and serve", my arse...