Because Iran is not threatening to vaporize New York, nor is it spouting about its intention to create a global empire by any means at its disposal and "as willed by Allah."
The Islamists are saying all of those things. An actual attack on the U.S. would prove the Islamists to be a global threat that must be exterminated.
Again I ask, do you think Osama bin Neitze has been secretly establishing detente with Russia and China? You think Russia and China believe the Islamists are really just A-OK, rational actors? Think they don't understand the economic and civil implications on their own turf of a nuclear-equipped Islamist regime, or even the economic consequences an Islamist attack on the U.S.?
Finally, Iran is a state with a big reciprocal target called Tehran. MAD at work.