An administrative subpoena under U.S. law is a subpoena issued by a federal agency without prior judicial oversight. Critics say that administrative subpoena authority is a violation of the Fourth Amendment to the United States Constitution, while proponents say that it provides a valuable investigative tool.
Well, the two are not, of course, mutually exclusive. A violation of the Fourth Amendment may well be, almost certainly will be, at the same time a "valuable investigative tool."
An important feature of administrative subpoenas, and especially in the case of NSLs, is that third parties--i.e., the persons and/or entities on the receiving end of these subpoenas--are subject to non-disclosure agreements: the very existence and issuance of these subpoenas or NSLs cannot be revealed to the targets. The validity of these non-disclosure agreements has been upheld in court--by the Ninth Circuit--as recently as 2017.
A second important feature of NSLs is that, like other administrative subpoenas, the issuing agency need only claim that the material or information sought is "relevant" to their investigation. In other words, there are none of the elaborate requirements and oversight schemes associated with that other premier counterintelligence investigative tool: FISA. FISA, as we know, requires probable cause of a crime, an elaborate approval process, and submission to a special court, the FISC. NSLs require essentially none of that--there is some oversight, on paper, but if FISAs are said to be "rubberstamped," you can imagine what the situation is like with NSLs. Basically, "revelant" is good enough and, like beauty, "revelant" is in the eye of the beholding agency: the FBI.
