There is...or used to be...
There is or was a federal law on the books that goes something like this: "Products purchased for government user must be bought from US companies." or something to that effect. So a list like this is probably classified, which means el Reg can FOIA it till they are blue in the face and they response will always be "We can neither confirm nor deny that any such list exists."
Frankly, I'm quite surprised they didn't outright ignore your request.
Biting the hand that feeds IT
