The official reasoning is that tobacco is addictive. I was in Edinburgh listening to a talk about censorship, in which the question was raised of exposing children to pornographic images. The speaker talked about how pornography stimulates (especially in children) a desire for more of the same. Satiation doesn't come into it, the mind requires more and more of the stuff – especially young minds that are not fully developed.
This does not happen with tobacco. At least, I am not aware of consumption of tobacco increasing to maintain satiation, in the way that can happen with many other things: alcohol, gambling, cocaine. Consumption remains steady, and while deprivation is irritating, it can be lived with.
Quite unlike the thirst to curb tobacco consumption. From Velvet Glove, Iron Fist (the byline): "When politicians start legislating for private behaviour they find it very hard to quit."