For subpoenas authorized under the Stored Communications Act, there's statutory authorization for DOJ to request time-limited NDAs, which makes me wonder if there needs to be explicit authorization for other kinds of subpoenas. This is the kind of noodling I'm doing here; I'm not trying to message-board my way to a first-principles argument that the NDA was bogus. :)
It's very common for a subpeona related to an ongoing investigation to include a gag order. For instance, if someone is investigating someone for a crime, and requests that users search history, the last thing they want is for Google et al to alert the user that this happened, as they may not be ready to arrest them yet and the target would flee.
Same with wiretapping orders, or frankly a subpeona for pretty much anything from a third party.
