The FBI policy is probably legal advice from FBI's justice counsel, and therefore solicitor-client privileged... meaning not subject to access to information requests.
This. Why would you even ask?
It's called a fishing expedition. You ask for the information and hope that you're corresponding with someone too stupid to know that it needs to be redacted. Most of the time, you're not that lucky, but once in a great while, you're accidentally given something useful. There's nothing lost in making the request (except maybe the cost of a postage stamp and the time to write a brief letter), and if you don't make that request, then you don't even have that slim chance of getting the information that you're seeking.
That's why you ask, even when you know that the answer is supposed to be 100+ blacked-out pages.