Japan assures U.S. that it doesn't plan on building nukes

I'm a bit confused. Given that North Korea has just detonated a nuke, and China isn't helping with the situation perhaps as much as it could (certainly not in the UN), and what China really doesn't want is a nuclear-armed Japan, why are we asking Japan for assurances that it won't develop its own nuclear arsenal? (See David Sanger's "Japan Assures Rice That It Has No Nuclear Intentions".)

Seems to me that that might be the kinda thing you just want to remain unsaid for the moment. Maybe a little strategic ambiguity? You know, maybe a little, "Oh, you know, I forgot to ask... What do you think, Mr. Zhu?"