I need to read those studies closely; but, I imagine there's a difference between gabbing about nothing and doing serious mental work when it comes to the amount of focus taken away by talking. Singing with the radio probably has similar focus requirements to gabbing. I have not done any tests on this; and we're not going to stop singing on the radio.
Fiddling with a phone is probably the most dangerous of phone related activities while driving, though. Looking and reading off the screen (texts), pushing capacitive spots on the screen to interact with it (changing songs), and so on.
Police most likely want eyes on the road, as even passive attention to the road is better than eyes completely off it.
[edit: replaced "be singing" with "stop singing" since that's what I meant to say]