Given that the US is looking towards non-OPEC oil producing countries as an alternative source of fuel, perhaps it does not want to further alienate the members of NAM, which includes not only Iran but also other major oil-producing countries such as Venezuala (6th largest oil reserves), Lybia (8th), Nigeria (10th), Angola (14th) and Brazil (18th)... Unless it wants to invade them too. But I'm sure a good bit of conflict investment goes on in those countries already.
The anti-proliferation regime will collapse if something is not done to prevent the perception of 'west versus the rest' - some things are seen as hypocritical, such as the western response to Irans nuclear program versus Israels. The problem is when national interests are what end up being furthered by the non-proliferation regime, not simple straightforward non-proliferation. The international institutions such as the IAEA need to be strengthened somehow if you ask me.