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The author is James Clapper, who Snowden's revelations proved lied in Congressional testimony about whether the NSA was collecting information on millions of Americans. So when he says that "multiple executive branch agencies, Congress, and federal courts [...] were all aware of and conducted oversight of the very programs that concerned [Snowden]," he's talking about the Congress that he lied to about these programs. How are we supposed to trust that Congress can conduct effective oversight when the intelligence community lies to them?
(We shouldn't.)
Snowden embarrassed the intelligence community and they won't forgive him for it. Americans (and others) should take that into account when they read or listen to the intelligence community's criticisms of Snowden.