>Your black Japanese student would be considered "African American", which is beyond absurd.
No they wouldn't, for the same reason Americans don't call Africans "African American."
No they wouldn't, for the same reason Americans don't call Africans "African American."
I don't refer to people primarily by their race, and I don't recall my friends doing so. While I don't deny that sort of conversation happens, it's probably not as idiomatic as you seem to believe.
I'm sure you can understand how easily a stereotype like that can fail when applied to a country as populous and diverse as the US.