Maybe they realized that Michelle was trying to make a political point, knew that the claim was untrue, and edited out the claim on that basis. I know it's a stretch. But, because most 0bama supporters are minimally informed, this sound bite would have reinforced the idea in many of their minds that automatic weapons are bad - regardless of whether or not they are owned or used legally. Regardless of the motivation, it seems to me a good thing that the erroneous claim was edited out.
I agree with you. The truth is best even if they find it by complete accident.