> How can any government accept a situation where communications are so secure that none of their agencies can break it?
A government by and for the people accepts it because the people accept it.
What a government _is_, of course, a philosophical question. The American Constitution treats government as an agreement among people, and structures that agreement to withhold particular powers from that agreement.
It does _not_ withhold these powers in recognition of some divine grant or innate possession of 'rights', but because it anticipates these powers make the government dangerous to the people governed. Given those powers, the people in government would inevitably abuse them, and would over time evolve into a faction or class to themselves, striving to retain control of the government for their own purposes and to the neglect of the people's.
A government that can consistently and without consequence exceed the powers allotted to it by the people governed will inevitably become an aristocracy, and a tyranny, unless those excesses are checked.
(There are other visions of what a government is. vidarh's comment on this subthread is the best account of anarchism I've read -- really smart -- saying that government is necessarily a class instrument that ought to be weakened or destroyed because it inevitably serves the strong economic interest.)
Of course, those limits make it harder for the government to pursue the people's interests. But the point is that those short-term deficiencies are a wise trade over the long-term, over which an unbounded government would transform into tyranny.
Surely the government can do a better job preventing the next 9/11 bombing if it has all these surveillance powers. But I'm not nearly as worried about such bombings as I am about how the government has already transformed beyond our control, and how much further it could go. I am very sure that 99.99% of us opposed to political terrorism can defeat the 0.01% using it, even while we tie our own hands in some ways for our own long-term protection. It's true that the technology of WMD have increased the potential leverage and destructiveness of that 0.01%. It's also true that we haven't hardly started thinking about how we, as a society, meet and check that expanded threat. I'm confident we'll figure it out. And even if we don't, the possibilities of that violent minority aren't nearly so dangerous or likely as those of an unchecked government.
So how can any government accept such limits? Because it is _us_, and we recognize that those limits best ensure our safety and that of our kids.