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.
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.
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.