Insurers are not charities. They price based on a risk model.
A police department with frequent liability payouts can expect their premiums to be priced according to the risk they represent (assuming they are even able to find an insurer still willing to underwrite them).
Do those risk adjustments actually occur in real life? Is it even specifically "police insurance," or is it under the umbrella of the insurance for the jurisdiction (city, county, etc.) where those risk adjustments can get buried within larger coverage patterns?