Is the only thing that changed in Obama's position his stance on security? If not, then I think there are several very simple explanations that might apply: * He didn't mean some or all of the things he said as a candidate and never intended to take action on them. * After he took office, he had to answer to more powerful interests who would not allow him to take action on campaign promises. * While campaigning, he promised things that he's incapable of taking action on, whether he knew it or not at the time.

Any of these explanations could be applied to any recent candidate for the office of PotUS.