You don't have to ascribe any altruistic features to the United States on this one. The US faces no existential threat to a conventional military. The other countries on its continent would never be capable of invading and conquering the US, and an oversea invasion is logistically impossible.

Nukes are the only real existential military threat to the US. It's therefore in our best interest that nobody has them. If that means getting rid of our own, then so be it. The US is perfectly capable of gunboat diplomacy without them.