I saw the plots once before and it looked fairly convincing because the arrangement of these seismicity lineations seems to be distributed geometrically much like the sea mounts to the east. I have also seen some of the most fantastic tomographic plots from Japan. If I come across any good web links I'll let you know where to find them. One explanation for this is that as the lithosphere sinks into the upper mantle it pulls some of the mantle above it down, which creates a sort of eddy current above the slab. The eddy then pulls the lithosphere above it along the surface towards the trench, thus causing spreading behind the subduction zone.
Japan has a smaller back-arc region behind it than the areas to the south.
But it does seem like it will be pulled further from Asia over time along with Kamchatka to the north. The rate of subduction under Japan is fairly high, and will probably persist for a long while to come. The lithosphere sinking to the north is older and cooler than that to the south, which might have helped to create the type of corner you are thinking about. It is interesting to think about back-arc spreading in terms of the western US when the Farallon plate used to go down on the west coast along the entire front.