I think it's because they have shown blatant disregard for all international norms of environmental care. The Iranians have a third world oil extraction regime, that floods toxic PCBs into the environment. Look at their record on chemical warfare - they regard the environment as a sort of toybox for their military to experiment with. FFS, they have been responsible for more deaths in the region from cancer than have died as a result of armed conflict in the last decade (don't believe me? Link here if you need the full details: Iranian deaths because of their casual attitude to industrial safety) So, given that a first world power like Russia could contaminate Welsh sheep with Chernobyl, we obviously cannot let this bunch of medieval peasants loose with the elemental power that drives our sun. I suspect that American rhetoric reflects the Bush administration's growing environmental awareness.