There is also a smoking ban in private vehicles that sometimes carry children.
It's bad enough that the children don't have to be actually present at the time you are smoking in your car for you to be arrested, but what if once, six years ago, you took your brother's kid to school? Does it mean you can never smoke in your car again?
What if the previous owner of your car used it to ferry little curtain climbers around town? Can you never smoke in it, even though you don't plan on letting children anywhere near it?