Probably because the US is not the police state many claim it is. Besides, they've done the fine job of finding holes in our intelligence services which allows the US to find and punish those responsible. I imagine it's usually difficult to find out what information 'spies' can access so as to try and fix those security holes.
To add to the discussion here's a link to The Economist's articles Son of SAM and Don't go back. Both are good reads. Also here's an interview of theirs with Julian Assange
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