None of this stops us restricting cigarettes to adults only and banning smoking in school, and it shouldn't stop us placing the same restrictions on junk food.
No, it shouldn't stop the regulation of "junk food", but some parents will say, "Why are you telling me I cannot feed my family this?", and food companies will obviously complain as well.
In my opinion (as a casual observer), the regulation of "junk food" will be nearly impossible.
And my answer to that would be, "I'm not telling you you can't let your children eat junk food at home, any more than I'm telling you you can't let your children smoke at home. I'm just saying I don't want your children doing either of those things at school, because I don't want other children to have to deal with a social environment where those things are regarded as normal behavior."
I don't want your children doing either of those things at school, because I don't want other children to have to deal with a social environment where those things are regarded as normal behavior.
This is literally no way in which this could backfire.