ramzey,
They watch for it because they're not looking for terrorists. They're looking for kids and "young adults" who want to play with dangerous stuff. At least in southern calif, that kind of thing used to result in forest fires, exploded garages, and kids with missing fingers.
Kids aren't usually all that street-smart, so you can catch the ones most likely to blow off their hand or burn something down because they often try to buy all the stuff at once.
