Why did the Japanese learn American English?

Probably, because it was an international language that used. Anyway, it's kinda common for asian to get to know about that.

Hmm, from my personal experience, some Japanese (grad) student was learning English because Americans (English speakers) had a jump on them in programming. He said that the many levels of humans in Japan made it more difficult to have a computer address a person. He also said that soon they would learn, and then they would be more advanced than us in programming. That was last century, and now we see that he was entirely correct — or was he?