"I believe it is a self-imposed sanction in their constitution, which they can change if desired. "
I think they're considering changing it.
I lived in Japan and the Phillipines when I was a kid...I loved Japan, great place, nice people, etc.
I loved the Phillipines too, but they remember all too well the occupation by Japan, as do many countries in the Far East.
The Japanese are a puzzling people. Capable of great beauty and appreciation of beautiful things, but seemingly capable of such cruelty.