It actually makes sense, this is not a matter of us defending Israel, but of not letting Israel make foreign policy decisions for us. They don't have anyone on the ground in places bordering Iran, we do. If they want to attack the Iranians, fine with me, but that doesn't mean we want to go immediately to 'Game on' with them ourselves. Locking into backing up anything they choose to do on their own is darkly reminiscent of how the Imperial German support of Austria-Hungary's aggressive response to Serbian terrorism dragged the world into cataclysm.
It's not really 'Defending Israel' when the question is whether to pile on whoever they choose to attack. If they do attack Iran, I'm sure we'll end up giving them plenty of actual defensive support for their own air defense and intelligence systems, but not active armed combat committing US forces overtly on their behalf outside Israeli borders.
BTW, this stance also gives the Israelis complete freedom to act on their own without having to take us into it (And risk Obama Administration leaking the strike plans), so it also in a sense greenlights their strike. It's all in the viewpoint of the people relating the story as to how one wants to take it, but strategically and diplomatically, it's the correct position to take.