If the public were treated like adults they'd wish for a nanny state.
I don't hear you arguing against the mobile phone ruling itself? Is that too going too far? You didn't say, but you did give a classic absurdist "slippery slide" argument, which can be applied to everything with equally silly implications. By the same "logic", a "Quiet please" sign in a library is a precursor to a totalitarian state.
All the examples you gave are already covered under the law "driving without due care and attention" which is applied, in the most part, with discretion. There's a safe way to have a conversation in a car (a passenger usually provides a 2nd pair of eyes). There's no safe way to use a mobile.
Yes, people acting immaturely require a nanny. Only children are forgiven for not understanding, or being answerable for, the consequences of their actions.
Advocating full personal responsibility, necessitates treating people as adults; with sentences to match to seriousness of the consequences. Death by dangerous driving entailing the same punishment as manslaughter by any other deadly implement.
As for cops doing it too; that's just the "two wrongs" reasonings. They shouldn't be doing it either.