The whole thing makes me itch because I like principles, and if there's a principle in the neighborhood, I can't find it. Maybe this is one of those areas where principles just don't work (are there areas like that?) and all that's left is gut feel, pragmatism, and guesswork.
How bad is surveillance? How much does limiting surveillance by requiring warrants make it less bad? If surveillance is bad, then is it totally non-bad if it's done to foreigners? Maybe there should be weaker restrictions on spying on foreigners rather than none? Does it matter to you if someone else's government is spying on you?