How do you know?
I regularly get downloads from Iran (and 3 other countries on Clinton's bigger list). Even if I wanted to (which I don't), I would find it difficult to stop these downloads. End-users frequently use small blocks of IP addresses which, on the face of it, are allocated to the US. I have to manually eyeball the reverse DNS to see where it ends up, and even that only works if something points back to a domain with a correct country code.