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Because there was other intelligence to be gleaned which was not the result of spying on Americans. Only the law firm was American; the rest of the parties were not. It's possible (likely?) that the NSA said, "give us everything with a non-American source" which would keep them within the law. I have not seen any evidence from Snowden that the NSA deliberately breaks the laws that bind them, though they make give the law liberal interpretations at times.