Michael Moore's perverse brand of fame may be peaking right about now, but for him it's still a win-win situation: he could ask for nothing better than to attract the attention and enmity of this Administration he's done everything he can to weaken. An aggressive investigation of Moore and how and where he got the film would be seen as proof positive of this Administration's repressive position on Free Speech, civil rights, etc. etc. This is why Michael Moore will be left alone.
I think you have it exactly backwards: the tape is win-win for supporters of the war effort. If it shows a pro-war Berg, then it undermines his father's hysterical finger pointing and makes him a martyr. If it shows an anti-war Berg, then it opens the door for questions about how he came into contact with the terrorists (and also demonstrates that he was about as wrong as could be in the process). The publicity fat boy gets from this-let's face it his audience is limited to lefty true believers and the French-is small potatoes compared to how the good guys stand to profit.