Earlier quoted context omitted.
I'm surprised the government hasn't just sent a banal request to everyone along the lines of, "you are required to disclose every known space alien who uses your service and you are prohibited from disclosing this order." Every service then has to kill its canary.
There would be no legal basis for such a prohibition, as there's no plausible national security endangerment were the request to be revealed.
However, I found this Quora answer to give pretty good justification on national security grounds for military scenarios:
https://www.quora.com/Why-is-the-government-afraid-of-reveal...