That's a real shame it would be nice just to say hey I'm an airband enthusiast and an air traffic control enthusiast and I would love to see your sectors and frequencies. But I understand their reasoning I guess post 9/11
And to be honest with you, there are facilities that would do it if they could. There is nothing super secret or anything on 99% of the material.
It's the 1% that they are worried about. It isn't even a 9/11 thing. There have always been classified documents in ATC facilities, just by the nature of the job. A great example is Letters of Agreement and Letters of Procedure with military entities.
What the FOIA process does is make sure that anything that shouldn't be released is either redacted, or just not released at all. When you file a FOIA request, the material does come from the facility itself, but it passes through other folks first, before it is released to the requestor. Generally speaking, the facility itself will not redact information or refuse to provide information (unless it's something blatant, like Area 51). They send off whatever they have per the request of the FOIA Coordinator. When it gets to the FOIA Coordinator, that's where things get redacted, documents removed, etc. Then it is released to the requestor.