Presumably because of FISA orders. Other countries have their equivalents of the NSA, but as far as I know none of them have a law preventing companies from announcing that their respective agency has grabbed a user's data.
It's hard to know exactly what the US law is too because it's classified. I would not trust that any country is safe from snooping--always assume it will happen.
That was what led the initial opposition to the Patriot Act, FISA Amendments, etc. was that they would make NSA programs like the ones that have hit the news substantially (if not completely) legal. That's apparently not what Sensenbrenner had intended when he drafted Patriot Act, but it was exactly this kind of issue that was raised at the time by privacy groups and promptly ignored by policymakers.
US law is very tied to court precedents, so a secret court with secret precedents is "secret law".
http://www.nytimes.com/2013/07/07/us/in-secret-court-vastly-...
> In more than a dozen classified rulings, the nation’s surveillance court has created a secret body of law giving the National Security Agency the power to amass vast collections of data on Americans while pursuing not only terrorism suspects, but also people possibly involved in nuclear proliferation, espionage and cyberattacks, officials say.
> But since major changes in legislation and greater judicial oversight of intelligence operations were instituted six years ago, it has quietly become almost a parallel Supreme Court
> “We’ve seen a growing body of law from the court,” a former intelligence official said. “What you have is a common law that develops where the court is issuing orders involving particular types of surveillance, particular types of targets.”
Etc etc.
The safe and conservative option is to assume that the executive is doing literally anything permitted by the public law without the aid of 'activist judges' (as the GOP would say) on your side of the argument. This is exactly why it is so important to be judicious in crafting legislation, as the judges will operate by what's in the written law not contrary to the Constitution when the law is clear. They only start worrying about "what the legislators meant" when the law is fuzzy.
That's also why it is important to quickly get a legal framework around foreign surveillance that includes recognition of the fact that the Internet is global while the Fourth Amendment is domestic. MUSCULAR is a pretty shocking breach of what we all understand the Fourth Amendment to mean, but I guarantee that it's technically legal. It shouldn't be, but the Fourth Amendment has long been known to effectively not apply at all outside of CONUS.