It's going to be cheaper for municipalites to have group insurance for this (or self-insure) than to have to pay the police enough that they can afford their own insurance.
The whole point of requiring individual insurance is precisely that insurance will be too expensive for people who are demonstrably high risk in that role, and less expensive for people who are low risk.
Some of the additional expense would be due to an individual risk profile, and some of the expense would be due to lack of bargaining power. The expense due to individual risk profile is a feature. The expense due to lack of bargaining power is not.
I don’t know if you’re familiar with how bargaining works, but you only get the price break if you can come in as a large unified group. Having millions of individuals doesn’t result in a price break. Eg There are millions of private individuals buying health insurance in the US, but they have no bargaining power unless they purchase as a unified block. Individual health insurance policies are notoriously expensive.
Ultimately it's the civil authorities and upper brass that want these intrusions. The insurance issue is easily worked around by hiring green recruits at a very high "bonus" to be used as basically burner employees to burn through their insurance and do the illegal stuff under their identity.
It has to be a criminal thing because the top brass and civil servants need RICO like prosecution and tossed in jail along with the guy who gets the insurance ding.
It’s already a (very real) crime to do a Conspiracy to deprive someone of their civil rights, which is what you’re talking about. Occasionally someone gets sued under it, but it’s rare.