It's kind of a harm reduction-style argument. The premise here is that banning smoking for a group more likely to already be addicted might be a non-starter, so

It's kind of a harm reduction-style argument. The premise here is that banning smoking for a group more likely to already be addicted might be a non-starter, so let's at least reduce harm where we can. A fairer way might be to say "people that don't already have an established history of smoking can never buy cigarettes going forward", but that immediately creates an unhealthy incentive for everyone to go out and buy their first pack right now.