AFAIK very few PCB- or chip-level electronics are made in the USA. And this kind of insertion would best be done at the last possible moment, at the factory. So if you don't have the physical factories, you don't get this opportunity. No doubt the NSA (or any national intelligence agency) would want this, but you can't have it unless you have the physical fabs.

So, being practical engineers, if I was the NSA I'd make deals with telecom's to 'assist' with writing the baseband. But the irony is that the device manufacturer could easily disable this backdoor in favor of their own. Oops! :)