The US policy since WW2 has been to discourage the spread of nuclear weapons, for self-evident reasons. This was done in part through self-restraint, in order to promote a nuclear weapons use taboo. If we started treating them as just another battlefield weapon, the number of countries owning such weapons would instantly multiply, until nearly every industrialized country had them. If every country felt that a conflict with the US meant it would be attacked by nuclear weapons, it would promptly take necessary measures to avert this possibility, including acquisition of its own nuclear arsenal.
Incidentally, the Nonproliferation Treaty to which the US is a party prohibits the use of nuclear weapons on non-nuclear states. This is part of the NPT bargain: countries forswear the acquisition of nuclear weapons in return for guarantees they themselves will not be targeted by such weapons, assistance in developing civil uses of nuclear energy, and a pledge by nuclear weapons states to work toward eventual elimination of their own weapons. You mess with one or more of the three, and you'll see more nuclear weapons states.
This is the paradox: the US is one of the few countries to have nukes mostly because these few countries have taken pains to act as if they did not have them.
Mod edit: I love this answer. its great, shows thought. thanks for posting it.
