I'm guessing it's because they honestly believe what they are doing is necessary to keep America safe.
This is like the banks and sub-prime lenders "honestly believing" that house prices would go up forever and money would always be cheap.
Read my lips: Everyone involved knew exactly what was going on.
Everyone inside the NSA with so much as a high school Diploma, when encountering even a low level program, knew that it was fundamentally wrong, probably illegal, and corrosive to the civic society. You don't even need to know what civic society is to know that tapping and permanently recording all calls in the US is both dangerous and wrong.
The on the record denials are effectively the NSA aping of the likes of John Corzine's claims of "We have no idea where the money is", despite being the man who took it right out of customers accounts. I dwell on the financial crisis because the breakdown in the rule of law, propriety, common sense, and all morality there is a mirror image and ultimately a fore-runner of the excesses and lies we now see in the NSA.
All that Keeping America Safe is BS. This is all about budgets, contracts, staffing levels, prestige and power seeking on the part of an entire city block of executives, officers, and IT workers throughout the NSA. The purpose of the NSA is to procure BMWs and range rovers for its management, and for favored private contractors and sub-contractors. That is why the price of a incorporated city is being spent on all these ludicrously overblown surveillance programs.
Forget the lies. Follow the money. Men will do anything, say anything, to anyone to keep such a gravy train flowing.