I believe DSP's main channel is left unencrypted so that tow truck drivers fire personnel, ect can hear whats going on. DSP tends to get dispatched first on accidents so its nice to listen to their channel and avoid the traffic. As rule there is nothing sensitive, DSP has a ton of other channels so they really have no reason to encrypt their main dispatch. Encryption also can drain a battery faster and adds complicated steps to an already complicated programming. I would imagine having multiple encryption keys for 2 separate agencys that may have each others channels programed could be a real issue as well.