^^this
No matter how you try to justify it, dropping a burning cigarette on yourself IS going to distract you and I'd rather my car (and maybe "my life" for that matter) not be on the receiving end. I don't have a problem with people smoking at all, I'm not a smoker but I sometimes have the odd social one in the past - I'd just prefer people to do it whilst stationary.
I completely see where people are coming from with the "ban everything" health and safety nonsense going on right now, but you're not allowed to adjust your sat nav etc when your driving, which wouldn't burn a hole in your thigh/your expensive retrim if you dropped it, so it makes sense to ban something that has the potential to cause injury to the driver.
The point comparing it to smoking in your house is academic, since your house doesn't tend to travel at 70mph+ or have the potential to cause death by the owner's distraction.