Much simpler. Encrypted radios are illegal for Joe Regular, per FCC*. Ergo, they should be illegal for law enforcement as well. Law enforcement should have no extra legal powers not granted to everyone, except for traffic stuff and the ability to arrest folks.
* Yes, they are. You can set up a packet network over the radio network (ie mini internet) and encrypt the IP traffic running over the unencrypted radio links. You cannot directly encrypt anything using most end-user frequencies.
Disclaimer: I am not a lawyer, and even if I was, I wouldn't be YOUR lawyer. Above is opinion, and not to be considered legal or technical advice.