An oft-repeated aphorism is training is "you will do what you were trained to do".
Seems the police (the few who were armed) were trained to only engage targets they are explicitly told to, and to account for every piece of brass ejected. Ergo, they waited for directives that wouldn't come, and were of the mindset that every round fired was a hassle to be avoided if at all possible - so, presented with a life-threatening target, they didn't shoot. They did exactly what they were trained to: nothing, pending orders.