A full ban would cause serious political problems

Yes they're selling it under the guise that one group is being "protected" because they ought to be protected and another group does not need this protection, which is a form of discrimination. They do this because they know they don't have the political capital to ban it for everyone, because many adults value the ability to make choices about their own health, well-being, and what vices they wish to partake in.

New Zealand has already enacted such a law, without seemingly much blowback.

It's pretty easy to enact laws on a small group when they have limited participation or voice in governmental affairs.

For what it's worth I'm not against this legislation, but I am critical of it. Big tobacco is bad. Cigarettes are bad. But this kind of "save the children" mentality often leads to a lot of corrupt and incorrect decisions and legislation. Alcohol is objectively even worse, and yet we're not banning that. Why? Will similar legislation try to capitalize on this and ban things like nicotine in general (almost certainly, despite nicotine being a relatively harmless substance in comparison). Worse yet, will they try to ban things like gender affirming care, or other objectively good things because there is a moral purity angle? I suspect so.