Trust us we have really good reasons and arguments but we just can't tell you. lol
I don't like the fact that they're doing it, but presuming that they should operate in total transparency just so that the silicon valley "proper thinkers" won't assume the worst is fairly naive, IMO. They believe they're fighting a critical intelligence war, and secrecy is an integral and necessary part of that battle.
I wouldn't work there, but I respect the people who do, both for their technical ability and for their patriotism. (I may not agree, but I can respect.) In particular, they're making significant individual sacrifices in order to do the work they do. It's not the same sacrifice as doing IED patrol in Afghanistan, but anyone doing high level crypto for NSA could likely make more money and have a cushier life in the private sector.
Where does the funding come from? From the tax payers. Who gets spied on? Everyone and their dog. So yeah, they're making a sacrifice, but even bigger sacrifices are forced on just about everyone else.
This would be a much more sensible argument if they weren't going around outright coercing Silicon Valley's tech companies into becoming their tools.
If the NSA wants to spy on everyone, they should do it the hard way!