The real reason for police agencies wanting encrypted voice traffic is to be less transparent, especially from the press. Modern government wants information to be dribbled out from the PR office.
The stuff that police organizations want to be obscurantist about are almost invariably not encrypted; dash cam videos, disciplinary reports, things like that. Yes, once upon a time some geeky reporters were able to get interesting stories out of radio scanners. That's because we were living in the dark ages, not because police tactical radio traffic should be open to anyone.
Informants have nothing to do with secure comms... the examples of reasons why secure comms are needed usually center around surveillance of organized crime for since 2001, the terrorists.
Is there a need for secure transmission of limited types of communication? Sure. But 95% of the radio traffic is not secret at all.
Remember CB radios? They were hot in the seventies with consumers -- and many of the bigger units had scanner functions as well. Any decent newspaper has had a scanner running in the newsroom since the 1960's, although the older models required you to set 5-10 frequencies using crystals.
Police radios are mostly used to dispatch and provide baseline information about incidents. There's no chatter about confidential informants, etc. Take 10 minutes, go to any of the scanner webcast sites on the internet and listen to the traffic for a few minutes.