Given that the guy was already in custody and immobilized, there were no longer any fears about recovery of lost items, nor any danger of harm from the perpetrator.
Given that it took 20+ minutes for this guy to die in custody, and given the responses of the guards, negligent homicide charges would seem to be justified here.
He was not in custody, per se, this is not a law enforcement issue. This was a private company subduing and recovering stolen property waiting on LEO to arrive at the scene. Now, if LEO had held this guy down on the pavement for 20 minutes that would be a different story.