hmmmm let me think you are 12,dumber then a bunch of rocks, and that is not the law.
as the op said, it is illegal to buy not to posses, the cops can do nothing about them smoking cigs back there, now if they are lighting some j's then i would call the cops.
but for op i would just confront them and ask them to smoke somewhere else, and just be cool about it and they should respect you, if not be an asshole to them.