“We have determined that the fact of the existence or non-existence of the (MH370) materials you request is a currently and properly classified matter in accordance with Executive Order 13526 ...”
They are not saying whether or not they have any MH370 materials because to even say whether or not they have any is classified.
I think there is a cat and mouse game going on here between the US/Aus/Brits and the Malaysians.
The Malaysians are in charge of the investigation and are doing the best they can to hide evidence like cell phone calls but what prevents them from doing so is not knowing whether the NSA has already scooped up those phone calls.
The Malaysians already tried to deny the co-pilot's cell phone connection to a tower in Penang, but then a NTSB official came forward and confirmed that the connection did take place.
Why didn’t Flight 370’s emergency beacon work?
http://edition.cnn.com/2014/04/25/world/asia/malaysia-airlines-flight-370-beacons/index.html