As with everything, it's those who take the P*ss that are promoting such changes in policy, I suspect.
Smoking on a straight road with few hazards to negotiate, and the window open, probably doesn't present a safety issue at all. I've done it enough when I used to imbibe. Most of the smoke is drawn out of the window and it's unlikely to impair your control of the vehicle.
However, I see so many people who get into their car and immediately spark up, with the window closed, and start trying to negotiate their way out of a car park and through an urban area while driving single-handed. That's a dangerous distraction IMHO, more so when they drop a burning ember into their crotch. Eyes watering due to the smoke and not in full control of the vehicle.
I even saw a van driver the other day driving through town smoking and using a phone at the same time.

If people behave like this we can expect consequences, unfortunately.
And yes, it's obvious that smoking in such a confined area with small children present isn't going to do them any good at all, so no problem with banning that, IMHO. Not that a ban will stop anyone, much like the ban on mobile phones.