The interesting aspect of NSLs to me has always been authentication. One gets a fax, and one faxes some crap back? Trivially hackable. One contacts the phone number listed on the NSL? Ditto. How difficult would it be for the Chinese or the Russians to slide in their own "NSL" in the 30K / year "legitimate" ones?

In fact the feds make it intentionally difficult to authenticate requests; for instance they prohibit taking copies of federal IDs, they often won't submit them for actual inspection, and they have no directory of employees to consult. If one wants to confirm that one is speaking to a bona fide FBI agent you're looking at minimum an hour in phone tag, and then there is the issue of if they are relating a bona fide request or going off the reservation.