Because it is generally accepted by the most powerful nations of the world (probably UN) that they can be trusted to use them responsibly (i.e. not attack america) whereas iran cannot.
in OP's sitch I would again enforce clearly to them that they are not allowed to poses nukes or materials which will be used to make them and watch them carefully and when there is entirely no doubt that they have them intervene. None of this gung-ho deal with "we think they have WMDs, we're going to wade into another war" which we so elegantly did before.