We have unencrypted police radio around here, but the most common thing you hear the cops say to each other is "Call me on my cell." Yay, transparency.
Certainly anti-transparency, but I'd say it's probably just easier for them. With a mobile (cell) phone call they can go hands free and talk in real-time in full duplex. That sounds better than waiting for the other party to finish and then pressing a button to talk to them... and using radio lingo.
