As a point of fact, Americans can legally buy a "fully armed attack helicopter". They typically don't as a matter of practicality and cost, not theoretical ability. The paperwork is pretty nominal.

Most Americans don't realize that, at least until recently, you could buy a kilo of C4 with cash, no questions asked. It is outside the experience of most Americans to buy high explosives retail, but that's actually a thing depending on where you live.