Yeah, neither Obama nor anyone in the White House made much of an attempt to defend themselves on the legal grounds. I'd say that there are two main reasons for this. The first one everyone has noticed; the president is, at times, a wuss, and I doubt, given the fact that it's an election year, this will be the last time this quality will manifest itself when faced with a wedge issue. The second is that this White House seems unwilling, in the heat of a political fight, to make what would otherwise be a clear legal case for themselves. They had a pretty solid legal basis for participation in the U.N. action in Libya, and they allowed themselves to get beat up over it for nothing. Now there's this too. It's very strange to me, because the president is a constitutional lawyer, and while I've no doubt at all there are better ones, I'm just as sure that he isn't the dumbest one on the block, so why he doesn't lean on that background when he needs to, and when it would otherwise serve him well, is silly.
Not strange at all. A legalistic defense may work very well in a court of law, but it seldom works well in the court of public opinion. The problems Obama faced on this issue were not legal, but political. Indeed, if the administration were to make the legality of the actions you mentioned the focus of their response, it would hurt politically. Better to respond with forceful defense of the legality of their actions only when directly confronted with it.
