It's really not, the justification is obvious; revealing information may very well also reveal how it was obtained. I understand the skepticism, and obviously this justification could easily be abused, but it's not some imaginary thing.

My wife worked in Navy intelligence for six years. She was privy to classified information. She, at times, knew where information was coming from, and it is obvious that the information itself would have implicated its source if it were to be released.

I'm not saying that's the case here. I'm pretty damn skeptical of my government as well. That said, the Fed is under no obligation to reveal all of the information it has. I think we'll be in the dark on this one for some time unless things change in such a way that would require more information to be released.