Easy Solution
I wonder what would happen if Google, Twitter, Facebook and Microsoft found that they were now liable for the content that their users publish?
For a start it would make them party to any terrorist conspiracy conducted with the aid of their services.
It's plausible that a terrorist attack will happen in the US, UK or Europe that is later shown to have been organised through Facebook, Google or some such service. Legally liable or not, the publicity would be fairly damaging to their reputation. Hypothetical headlines such as "Google Helped the New 9/11 Happen" would probably spoil Schmidt's breakfast.
Ok so that's a hypothetical situation that may never happen. But if it ever did happen the commercial risk to such a company is staggeringly high. The public would certainly have an opinion that someone should have done something to stop it happening. Google, Facebook, MS, etc. are by their actions all inviting that "someone" to be perceived to be themselves. It would be very difficult to avoid:
Google/Apple/Facebook/MS/etc: "We're encrypting everything so that the NSA can't read your stuff"
:BANG:
Public: "Who knew this was going to happen?"
NSA: "Not us, no one lets us read anything these days"
FBI: "But we did find this incriminating content on the terrorist's computer and in his Google account afterwards"
Google: "Er, uurrrrmmm..."
Lawyers for the victims: "I invite you to respond to this law suit"
Apple/Facebook/MS, sotto voce: "Phew, it wasn't us, we're not involved"
FBI: "And it turns out he had an iCloud account too which we want to look in"
Apple: "Crap"
To avoid that the companies need to be seen to be in a position where their content has been adequately policed.
Arguably, pre-Snowden, one could say that the content held by these companies was policed for them for free by the NSA. Never mind the legality of it, in effect that is what was happening (though who knows how effective it was?). Post-Snowden, Google, MS, etc. are saying that they'll be encrypting everything and the NSA presumably are having difficulties carrying out that policing. I'm guessing that Google, Facebook, etc. aren't doing it themselves. That would be like they're taking a commercial bet that it would never happen to them. Which in today's world environment is a little bit like sticking their fingers in their ears, shutting their eyes and going "la la la".