The term African American...

Kr
Ok, bear with me on this, but it is something that I have honestly been curious about for quite some time and haven't ever had the opportunity to discuss in such a diverse place like good ol CC.
I do not not understand the usage of the term "African American" when it comes to referring to a black person who is not from Africa. I have noticed that African American is a blanket term for any black person and it seems to be what the majority of people think is politically correct, but I cannot understand why.
Unless you were born in Africa, you aren't African. You may have ancestors or be a descendent from Africa, but just because I have family from Germany, I don't call myself a German American. Nor do other white people from other countries. And I haven't noticed this trend with any other races, unless that person was born in a certain country/region and moved later in life to another country (Asian American, for example). 
What about black people from the UK, Haiti, heck, Mexico if that is where they were born? They aren't African American, but they get "lumped" in that category when describing them. 
I hope what I'm confused about, for a lack of a better term, makes sense and is coming across the way I mean it to. I also hope it goes without saying I'm not at all trying to be disrespectful or "ignorant"...I just truly don't get the concept.