I don't think he's falling back on anything. I think he and Bush share this in common. It's the amorality of pragmatism. I think he takes very very seriously his role as protector of the nation and isn't going to allow morality or moral ambiguity to get in his way.
Except that's not his job at all. All presidents make this affirmation, as inscribed in the Constitution itself-
"I do solemnly swear (or affirm) that I will faithfully execute the office of President of the United States, and will to the best of my ability, preserve, protect and defend the Constitution of the United States."
That's much deeper than merely doing what's represented to be expedient to "protect the nation". The constitution provides the framework in which the president can act- the framework of the rule of law. When and if he acts outside that framework, he's not defending the nation at all, but rather attacking its foundations.
GWB willfully acted outside that framework in the establishment of the prison at Gitmo, acknowledged such at the time. And it wasn't done to protect the nation at all, but rather to gain domestic political advantage. The terrorist threat was completely blown out of proportion to serve as a bludgeon in affairs foreign and domestic.
Obama's rationale is similar in the sense that he extends the situation so as to protect his political position rather than the nation and the constitution he swore to serve. He has some justification in the sense that the law was changed in the meanwhile, but it remains obvious that the constitutionality of those changes are very much in doubt.
Meanwhile, our govt holds prisoners against whom no case can be made, apparently, not even in a court of the kangaroo variety. Whatever evidence there is remains secret and inadmissible in a constitutional court of law, 8 years later, if there really is any at all.
Even more astounding is that people who otherwise have a more than healthy skepticism wrt govt in general support the practice, accept as gospel the pronunciations of the people who perpetrated the whole thing as a way to achieve their own ends, which really had nothing to do with protecting the nation at all.