Many people who don't speak English are outside the Americas, and so it doesn't make sense for them to learn the American variation of English. However, I agree that it should be an option.
(I prefer South African English, personally, but I think the commercial market is strongly in favor of American English.)
"No, your anecdotal evidence is wrong."
"My anecdotal evidence is right."
(A totally fluent UK/IE/etc English speaker probably is fine in any context, but I don't think someone learning from a webapp is starting from that point. If you are going to be a moderate speaker, it is better to be moderate in the target accent.)
More anecdata for you.
Same thing in China and Thailand; I don't know about other countries.