arma virumque cano (et alia)
It's a reflection of the fact that they're innocent until proven guilty, while assuring the public that [allegedly] trigger-happy cops are removed from the line of duty. It's also important for the mental health of the police officer to have some time to assimilate (Not invent) events and receive mental health treatment.
Yes, they don't need time to invent anything. They learn very quickly that when it's a knife or bat, the key word is "lunged." When it's anything else that could be mistaken for a gun, it's "pointed."