Apple's data on requests by law enforcement
indicates that the company is relying on a practice known as a "warrant canary," ArsTechnica
observes. The concept involves publishing a notice that a warrant hasn't
been served, and simply omitting/pulling the notice if the opposite is true. This can be a way of getting around gag orders that prevent organizations from disclosing their compliance with government surveillance.
In the case of Apple, the company states
that it "has never received an order under Section 215 of the USA Patriot Act," and would "expect to challenge an order if served." The company doesn't, however, mention Section 702 of FISA (the Foreign Intelligence Surveillance Act), which can be used to pressure companies into sharing data on foreigners. 702 is also the legal foundation of the National Security Agency's PRISM program; the omission of a notice on the matter may therefore be a quiet admission that Apple has handed requested data over to the NSA.
Many other high-tech companies in the US -- such as Facebook, Google, Microsoft, and Verizon -- have been faced with public scrutiny over their compliance with PRISM, voluntary or otherwise. In some instances the parties have fought to publish more data on law enforcement requests, but have been limited in what they can say by legal barriers. Disclosure's such as Apple's are generally considered a way of improving public image.