Talking on a mobile is distracting and dangerous, if you are holding it to you ear, not only have you only got one hand on the steering wheel, you concentration is not fully on the main task at hand.
That written, you're not supposed to eat while driving, people can get stopped and prosecuted for eating or drinking while driving.
So why can people smoke while driving without any legal comeback?
Which is worse IMO, if an emergency occurred which you needed to react to, I'd think nothing of dropping my Mars bar or sandwich on my lap, however, what you gonna do with a lit fag?