I hope they realize this is all the more reason we need to see it. Clearly the Nunes memo alleges serious wrongdoing on the part of the FBI in obtaining the Carter Page wiretap, and the FBI is obviously worried that when the public reads it they will find the memo damning.
This is the same FBI and DOJ, remember, that stonewalled a legitimate congressional subpoena of the relevant documents for five months, agreeing to turn them over to the House Intelligence Committee only when threatened directly by Paul Ryan with a contempt of Congress resolutioon. Even then they only agreed to let members of the committee view the documents and take notes, not keep copies. That's the only reason Nunes had to write the memo in the first place. He had to memorialize what he'd seen because he wasn't allowed to keep any of it.