That may well be true. But they're not the only ones who fear a nuclear Japan, and all the other regional powers will look to Beijing -- not Washington -- to ensure regional security in the face of what they will perceive to be Japanese aggression. That's, imho, a grave geopolitical mistake.
Additionally, a nuclear armed Japan will legitimize everything the DPRK is saying -- that their nukes are purely defensive, and make it impossible for the US to build the kind of international resolve necessary to enforce an effective embargo of the DPRK. The US needs Kim Jong Il to remain the bad guy -- how he's currently viewed. If Japan builds nukes, the children and grandchildren of victims of the Greater East Asia Co-Prosperity Sphere will start seeing Japan as the bigger threat.