Simply put, the U.S. was likely to adopt UN regulations in an attempt to help ease the global shipping process by adhering to one uniform policy. The problem with shifting to UN regulations is that there is no "ORM-D" status, so if/when this happened, ammunition would have to be shipped under the UN 1.4s category – a category that includes HazMat fees.Would you have liked to pay HazMat fees on any ammo you purchased mail order? How much do you think the ammo at Wal*Mart would go up if it all had to ship as HazMat? But SAAMI stepped up to the plate, lobbied hard, and got ammunition exempted.
You'll note absolutely no mention of the National Rifle Association in this story. They do other things.