[uoou]

"I can imagine that opinion between pro/against varies depending on nationality"

While I agree that that is true, as a non-US-citizen I also find that slightly offensive (not your saying it, but the fact that it's true). It's unambiguously wrong to invade your citizens' privacy, but it's ok if it's one of us?

And furthering ancarda's point, maybe it's a more interesting questions when it's more abstract:

Would it be ok for an intelligence agency to monitor all communication for criminal activity, so long as they only act on the information they gather when criminal activity is indeed taking place (and let's assume they always act, there's no discrimination. And they do not use the information for any other purpose).

If an intelligence agency has the technical capacity to monitor all communications then there'd have to be a pretty compelling reason (assuming their raison d'etre is the prevention and detection of crime) not to. They'd just be doing their job to the best of their ability, it'd be negligent not to.

While I would of course not be comfortable with that, I've never actually heard a convincing philosophical argument for personal privacy.