Don't be misled by an assumption that, because it's Ames, it has to have come from the U.S. That's like saying that Spanish influenza can only exist in Spain.
The Ames strain was shared with several other countries -- both as a biowarfare possibility (to military labs) and as a serious animal husbandry threat (to vet med schools).
Thus, the sample that was cultured in volume by the Iraqis could have come via any of many possible sources. And it was chosen for weaponization, not because of its American origin, but because of its virulence.