The current debate over acquisition of foreign intelligence takes many things for granted, so much so that many key concepts are neglected. Aong these are:
1.) The nature of privacy. The notion of privacy is given great deference in national security debates, so much so that it nears divine reverence. What is lost in this treatment, is the fact that privacy has a purpose. Citizens of a free society need to have some measure of control over their own reputation. It is necessary for the orderly interactions that are necessary to living with one another. This is why English common law allowed actions for libel and slander, and why the common virtue of minding ones own business has survivied as an element of good manners. A reputation is no small matter in any community, and respecting privacy makes it easier for one to defend against scurrilous acusations or other stains upon an individuals honor. Society should therefore respect privacy because it benefits the society to do so, not because it is an absolute virtue. A society is not required to allow plots, even by its own citizens, to advance in darkness, simply because "privacy" is beneficial in other respects.
There is no benefit to American society in allowing non-citizens outside of national borders to transact any business, no mater how innocuous, in secrecy. Respect for the privacy of such transactions is not a matter that affects the orderly business of this society, and there is no infringement on any right by simply slistening on on what there is to hear. The right of privacy inherent in the fourth amendment is a compact between a government and the free people that consent to it, not a capitulation of a universally recognized human right.
2.) "Expectations of privacy." Even in American criminal law, privacy rights are subject to the common sense limitation that there is no privacy where none would be expected. Two non-citizens outside of the United States cannot reasonably expect that a communication that passes through the U.S. would not be scrutinized by a government that is trying to protect its people from nefarious schemes and terrorist atrocities.
3.) The concept of penetration in terror attacks. When terrorists plot an operation, one of the most difficult elements is to determine how to penetrate the vigilance of the target to accomplish the deed. This is an element of all terrorist operations, from the bombing of the Marine barracks in Beirut, to the attempted assassination of Margaret Thatcher, to the attack on the USS Cole, to the attacks of 9/11. Penetration is the most tenuous element in a terrorist plot, as can be seen in foiling the Millennium plot. Thus, this is where terror plots are most vulnerable, and where surveillance is most useful. Surveillance is a very efficient way to defeat the ruses and stealth used by terrorists to penetrate their targets.