I submit that there is no technical (legal) reason that one or all of these internet companies can't boldly disclose this information. But there it is - the $64,000 word - "boldly". The practical reason is fear. When we have an executive branch that acts as described by Mr. Murphy (and it's probably worse), what's to gaurantee that there is no damaging response? So, I see this as this is a political attempt to deal with their fear.
But the irony is that most of these big tech companies - and the people who embrace them - supported this and then voted for this.
I'm not sure of the legal details, but basically the NSA prevents them from sharing anything about what the government is seeking. The Web folk want to be like the injadvertant spies who came in from the cold.
As a baby boomer, I'm part of a generation that worshipped the notion of privacy and deeply feared the idea of a government that compiles dossiers on its citizens. We identify that sort of behavior with Nazi Germany, the East German Stasi, and Stalin's Soviet Union.
The younger generation isn't like that. They barely remember the horrors of the 20th century. They generously divulge everything about their lives, their politics, and their associates without a care.
Perhaps the young are right, and times have changed. But that is not the message I get from the US government these days as it prosecutes journalists, hunts down those who disclose sensitive info, and spies on its allies, enemies and its own citizens.