Linus doesn't review everything. He delegates and trusts. That's tangential, though. More importantly, I expect NSA contributions to be poured over because the

Linus doesn't review everything. He delegates and trusts. That's tangential, though. More importantly, I expect NSA contributions to be poured over because the NSA isn't highly trusted, and it would make a great mailing list post to say "The NSA has a backdoor in our code here, here, and here." Many eyes and a suspect contributor make all backdoors shallow.