It actually was the President of the ITC who vetoed it, not Obama. The President of the ITC determined that Apple filed to license those patents from Samsung but instead, Samsung wouldn't license them without requiring Apple to give them free access to some of theirs. Since this is obviously not a universal requirement and Samsung specifically wanted to make it harder for Apple to license them than any other company it contracts with, the President of the ITC decided to Veto the ban and call the abuse in favor of Apple.