The limiting principles seem to be encoded in the law[1] that provides the basis for most of the government's arguments - FWIW, passed by both houses of Congress and explicitly granting this discretion to the executive branch.
Most of the recent furor seems to be around whether the NSA is actually complying with (d)(1)(A) and (B) or is just gathering up all of our information without the court order required for surveillance of any U.S. citizen inside the U.S.
Most of the recent furor seems to be around whether the NSA is actually complying with (d)(1)(A) and (B) or is just gathering up all of our information without the court order required for surveillance of any U.S. citizen inside the U.S.
[1] http://www.law.cornell.edu/uscode/text/50/1881a