This is one untypical case that makes the news, so you're thinking about optimizing decision making for this case; all the cases where police actually get shot don't.
In war, soliders are required to have an extremely high degree of certainty before opening fire. I think it's pretty safe to hold a higher standard to a police force that is dealing with primarily innocent civilians.
If they don't want to risk their lives, they can find another job. That may sound unfair, but it's more unfair to kill and murder someone who poses no threat.
That applies for business and software in addition to morals...