If I'm not mistaken, all of Japan's royalty, the Imperial House of Japan are claimed to be descended for Amaterasu. Not just Hirohito.
Am I wrong? Also, what's so nationalistic and wrong about this religious idea?
Exactly that claim plus the supposed to be "bloodline" was built up for the followers and to gain national strength. Its undertandable in the whole context though, and a clever idea.
I do like the Amaterasu idea as such (only concerning Amaterasu and Co.) in general though, very much so even, and often try to explain it to friends with joy, but independant of all that followed, and just as an image. A fine one indeed! No doubt at all.