Despite how I'm 100% against smoking in all its forms, as I don't see what the point of it is if it has zero positive consequences...and how I think smoking itself should be illegal in public (second-hand smoke).....I think this rule is a little stupid.
A 30 year old guy buying cigarettes with his daughter, refused because his daughter was with him and he MIGHT be buying them for his daughter....now, in America, you can buy cigarettes at 18. I've known lots of people that had just turned 18 the day before and they were already buying cigarettes for tons of kids that weren't. This I can understand...if you saw some 18 year old kid with his 17/16/etc friends, they all seemed like the same type that would be smoking, and it just feels like the situation where the other kids that weren't 18 would be using them. So maybe they made up this rule as a "generalization" kind of thing just to cover all bases so nobody says "well you didn't do it to the 30 year old guy with his 14 year old son".
Either way, though, kids that want to smoke are finding ways to get the cigarettes, so this won't help one way or the other, it'll just piss people off. After putting it into effect, the new loophole will just be to wait in the car while your friend buys them.