It really would be better if you'd forget the passive voice. "Is allowed" supposes some higher authority either allowing an action or not. The US is a sovereign nation. There is no higher authority to allow or disallow any US action, though the country, like any other, has to weigh the good against the harm of an action. In this case, the use of nuclear arms would doubtless be fraught with negative consequences politically. At any rate, the US military is moving away from nukes as part of its military strategy, on completely pragmatic grounds: nuclear, biologic, and chemical weapons aren't needed, because we have such overwhelmingly effective weapons without using any of those.