Unless they're broadcasting "just planting some drugs on this suspect, over", having the transmissions unencrypted probably isn't a very useful way of auditing police activity.
On the contrary, there have been several high profile incidents where police radio traffic was used to demonstrate ill intent. It's difficult to use a tool day in, day out like that without getting complacent.
Open radio also makes it more likely the news media is there when the action is still happening instead of giving cops time to paper over their mistakes.
The point is that deployment of police power to a particular location/situation becomes public record immediately and unconditionally, and the press can (and does) follow up on it.
Police getting to operate in secret is a dangerous road to go down. Same reason that the names of suspects who get arrested are public. It might suck for their future reputations, but the alternative is a world where the police get to make people disappear.
