It's a part of the Patriot Act. This is from a Wikipedia article on a warrant canary, which is a clever method devised to alert customers of such warrants indirectly. [1]

> Such subpoenas, including those covered under the USA Patriot Act, provide criminal penalties for revealing the existence of the warrant to any third party, including the service provider's customers.

[1] http://en.wikipedia.org/wiki/Warrant_canary