Go to this thread to debate.
http://www.freerepublic.com/focus/f-news/1543216/posts
And you need to read the entire statue. Now I don't know how you can believe that this was an illegal act, but I'm not going to debate you. One final note: the term US Citizens does not apply to people with temporary visas. That was what the "wall" was all about and how Able Danger was stopped.
But, since you are not going to debate the point you yourself raised here, there's not much point in asking you what part of this statute allows warrantless surveillance of US citizens. But, there wouldn't be much point anyway, because it ain't in there.
bobsunshine's statute is plainly not applicable to surveillance of US citizens. Posts justifying this program under section 1802 are DISINFORMATION, and nothing more.