i should have been less ambiguous, sorry -- USSID 18 is derived from EO 12333, and USSID 18 nominally provides protections to US citizens from being spied upon; USSID 18 also is what has a secret interpretation by NSA lawyers that most people would probably feel is not in the spirit of the EO. here's a very nice writeup:
Why don't any of Snowden's documents confirm that they're using this interpretation and instead show that they are interpreting it as everyone else does?