Althtough it's even sillier to call someone an 'African American' when they live in another country, such as the UK.
That's not standard practice, though morons who haven't thought about the term have been known to do it. You have to be American to be African-American.
I think the term is poorly used in almost all situations. Those are landmasses, not genetic backgrounds. Check it:
Black-skinned person born in America, whose family six generations back were born in America: Not African-American.
Black-skinned person born in Cuba who immigrated to America: Not African-American.
Black-skinned person born in American who immigrated to Australia: Not African-American.
White-skinned person born in Nigeria who immigrated to America: This one is an African-American!
I suspect it came about as a politically-correct way to refer to black people, probably a result of census use and popularized by the police and military jargon. You know, like "I arrived on the scene and witnessed one male African-American individual and one male Caucasian individual conversing." It's just a thing people blurt out of their mouths hoping nobody gets offended.
Also people like to hold onto their ethnic backgrounds despite transplanting to a different culture. It must be very hard to immigrate somewhere where nobody speaks your language or uses your religion or customs. I think it's why ex-pats like to band together into sub-communities.