> t seems pretty obvious to me that the police act partly according to how they were trained,

Not doing so is grounds for dismissal in a lot of precincts. It's why we hear stories of officers doing the ostensibly "right" thing by not killing a suspect, yet face dismissal. The department training guidelines dictate a use of deadly force in a scenario, and not doing so is seen as risking the lives of officers.

This is the hill American need to climb to reduce the likelihood of death and serious injury by police. There is a lot of judicial precedent supporting such policies as part of policing guidelines, so it is very unlikely the change will come from the courts. Instead, citizens will need to work at a local level to shift training and policy towards de-escalation.