Here is a possibility that I don't think many of you have considered. The autopsy report says he had bruises and contusions on his head. The cops did not beat or hit him in the head.
It is my understanding that he was thrashing around in the back of the police car, so they dragged him out through the passenger side and held him down.
Could their intentions have been to hold him down so as not to further injure himself from his thrashing?
There are other ways to control someone in cuffs than kneeing on his neck.