He did this so there wouldn't be any question to the legality of his presidency over a few misspoken words. Of course the people that would bring up such a pathetic charge have already said that this 2nd swearing in isn't valid because he didn't place his hand on the bible (which is not required). CNN is also pissed about the 2nd swearing in, because the media wasn't allowed to film it. There was only audio and video this time.
The whole thing is ridiculous since come Noon on 1/20 according to the constitution, oath or not, Obama legally became president.