Export Controls.
It is illegal to export guns or ammunition to countries like North Korea, Iran, China. If you consider some of the more specialized items for sale at stores Dick’s, like high powered scopes or armor piercing bullets, the list of prohibited countries gets a lot longer.
The way that export control laws are now written, selling a prohibited item to a national of a foreign country within the U.S. is considered an “export” of that item. So if Dick’s Sporting Goods sells a rifle and ammunition to an Iranian citizen, they are probably in violation of export control laws. And I am guessing that ATF or other agencies would love to hit a major gun/ammo chain store like Dick’s or Cabela’s or Gander Mountain with huge fines for violating export control rules.
In my opinion at least, your anger is misplaced. Dick’s is just trying to follow the law and stay in business.
Ever consider that many (Most) of the laws concerning the 2A are unconstitutional?