It's a self-perpetuating feedback loop. There was panic buying after Newtown, and once reports of shortages popped up, even more people panicked and went out to buy ammunition, leading to more shortages, more panic buying, and so on. The silly panic over DHS ammo purchases (but not the small purchases themselves) has increased the phenomenon even more.
For gun owners, the answer to "why is there an ammo shortage" can be found by looking in a mirror.
Your explanation is correct, but a lot of people aren't going to want to let go of this "great DHS ammunition stockpile" conspiracy theory. The Alex Jones crowd, in particular, is going to be regurgitating this conspiracy no matter how clear the evidence is that it's a myth.
You’re right, but as long as there are reasonable FReepers who debunk the kooky hoaxes, this site won’t become a PrisonPlanet clone. It may take a while, but the tide usually turns against the falsehoods if they keep getting answered with truth.