Teenagers use spray paint to write graffiti. Several cities in the US (including my hometown of Philadelphia) have banned the sale of spray paint to minors because of this.
Screw your opinion on that. My friends were given loads of crap for buying something like ten cans of spray paint at the beginning of the school year. Guess what it was for? Painting their names and whatever else they'd like to on their parking spots per school rules; they are in the SCA. Sounds like graffiti to me.
Admittedly, I don't know if my current place of residence (New Haven, CT) has such a law on the books, but I assure you that these kids weren't buying those cans to work on their soap box derby racer. I suppose you'd also think it okay to sell them dozens of boxes of cold medicine so that they can turn it into meth. Or liqueur. Or guns. Hey, it's what they want to buy, right?
Honestly, what is wrong with you? Are you having trouble in your life or something? This lashing out at teenagers is rude, ignorant, and close-minded. Far more adults use meth, drink liquor, and use guns than teenagers. I don't know the teenagers who bought the cans near you, so I can't really defend them. However, if your opinion of them is as skewed and ridiculous as your opinion of teenagers in general, I'd guess that you're wrong.
And who knew all I had to do to get myself more liquor and guns was to stop buying action figures. After all, it's the former that I purchase, according to you.