If one of Moore's minions interviewed Berg, then the footage needs to be seen by those investigating the murder. It could contain information of his plans, his contacts, etc.
Moore is hiding it because it also will reveal who took the footage.
This is the most rational, intelligent point on this thread.
We all hate Moore, but there is no evidence to believe that Berg was doing anything fishy in Iraq other than his naive going there in the first place. And Moore is obviously using the fact that he has this tape just to grandstand: by now the echoes of his standing O in Cannes are fading and he needs a little more limelight.
Lucky for us that he does, because now the authorities know that it exists and can subpoena it for possibly useful info leading to his murder.