They're not "racist", they're just "too lazy" to change any of there laws since the 1960s, aside from the recent CP thing a decade ago.
Anyway, I looked it up:
No, the child will likely not automatically be granted Japanese citizenship if the parents are a naturalized Japanese citizen (who was previously American) and an American citizen. While the Japanese Nationality Law allows for naturalization of a child born in Japan to stateless individuals, this doesn't apply in this specific scenario.
Naturalization:
Naturalized Japanese citizens lose their previous citizenship when they become Japanese.
Dual Citizenship:
Japan does not allow dual citizenship.
Possible Statelessness:
The child would likely be stateless at birth as they do not have Japanese citizenship and their American parent's US citizenship wouldn't automatically grant them US citizenship either.
Statelessness Exception:
There's a specific exception for children born in Japan who would otherwise be stateless. They can apply for naturalization at age 3.
But I did find this: Baby born to Afghani refugees in Japan granted Japanese citizenship, probably depends on the court.
https://www.reddit.com/r/japannews/comments/1ffohxp