The letter reminds me of how the DEA is consistently hyperbolic regarding the public health dangers of marijuana. To concede that weed is essentially harmless and medically useful would invalidate much of the drug war the DEA has waged for the past 40 years.
Similarly - to acknowledge Snowden in any positive light would invalidate the spying regime instituted by the U.S. government. And force the to acknowledge that their powerful spy tools require equally powerful oversight.
The lesson the NSA, the House committee, and others should take from Snowden is not that he's a villain. Rather, that one way or the other substantial oversight is going to occur, even if it requires the largest leak in history. Create truly transparent and accountable systems and there will be no more Snowdens. Throw the book at him, become more insular and secretive, and the problem will only get worse.