How many of those are there really? How much are you willing to sacrifice to prevent this hypothetical? Because there is no end to this sort of argument. Why should it stop with foreigners, wouldn't a suicide bomb by a local cause just as much damage? "We have to monitor every web page and every email, text, and word spoken of every person at all times to 'prevent a tragedy'" You want to prevent a tragedy don't you? You don't have anything to hide right? The fact that these kinds of powers have always been abused by those who have them is not something you should be concerned about. It won't happen to you. They will only go after the bad guys, and you're good, right? Now show us your papers.