NSLs typically contain a nondisclosure requirement, frequently called a ''gag order,'' preventing the recipient of an NSL from disclosing that the FBI had requested the information.
...however:
The nondisclosure order must be authorized by the Director of the FBI, and only after he or she certifies "that otherwise there may result a danger to the national security of the United States, interference with a criminal, counterterrorism, or counterintelligence investigation, interference with diplomatic relations, or danger to the life or physical safety of any person.
So it seems this is regulated to some degree, though it is unclear what counts as "interference with a criminal, counterterrorism, or counterintelligence investigation".