I think he will be exempt since he was trying to make a point for the "Ban Crowd". Clearly he was in possession (if he was a no-name teenager on the street and the mag was weed he would surely be arrested) and he was "brandishing" the mag on top of that. Luckily no one was injured.
Based on the latest Yahoo News Article is looks like he won't be exempt.
It appears NBC asked the DC police first if they could use the 30 magazine round in the segment and the DC police said NO as it's illegal to have such a magazine in DC. NBC went ahead and did it anyway.
So they can't claim they did not know it was illegal.
Did TV Anchor Violate Gun Law? - Yahoo! News