I don't consider a nuclear armed Japan to be much of a problem. Let's face it - we mess around with them, but the Japanese know soul deep exactly what even a small nuke does. They are still traumatized about it - rightly so.
If they need them, they need them. They have had the tech capacity to make them for years, and only their insistence on sticking to the peace treaty has prevented them from producing them.
However, Obama's "pivot to the East" means the US needs to keep it's hand in that cookie jar. If your major allies in the region can look after themselves, US influence is very much diminished.