i don't know why you're all stumped on this. Moore filmed an interview with Berg, who was an American in Iraq. Berg was not sufficiently anti-American policy, so it was not used in teh film. Moore and teh Berg family do not want America to see the footage because it would now be viewed as a final pro-Bush testament from a murdered American. Moore created the footage, he did not get it from terrorists. This is a cover up and Moore is a left wing scum bag.
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.