I cannot answer Thomas Berger's question. I too have wondered about this. The first reference I have seen to Japan needing to "Amerika ni hairyo" came about two weeks ago in a Yomiuri article about Japan (allegedly) going to zero nuke dependency in the 2030s. It was a very off hand statement in a rather long article (about two-thirds of a page). No explanation was given.
Since then, this mantra has increased in frequency and prominence. I have yet to see anything remotely resembling an explanation of what is behind this.
Moreover, I don't recall any references to the US in the German decision to pull out of nuclear generation.

When I first saw this assertion, I immediately wondered whether there was some covert arrangement by which Japanese nuclear fuel recyling would be providing the US with an alternative source of weapon's grade nuclear material. I thought this because much of the Yomiuri article was about how going to zero would impact on the Japanese recyling program.

My own personal feeling is that this is probably "made in Japan foreign pressure" (Wasei gaiatsu). We've gone through a relatively hot summer with almost zero reliance on nuclear generation and virtually no concerted power saving efforts. Many of the local shops are still air conditioning the street. Despite signs claiming they have upped the temperature to save power, many large stores are uncomfortably cold.
Commuter railroads seem to be running a full schedule with air conditioning at pre-Fukushima levels. In this context, the nuke lobby does not have anything concrete to point to as the dire consequences of (near) zero reliance on nuclear generation. Hence, the "Amerika ni hairyo" mantra.

But, this is only speculation on my part. I too would like to know if there is any substance sinister or otherwise behind this.

EHK