White people do get that question, I don't deny that. The problem is that American history is riddled with racists whom associate "true Americans" as "Americans born to Americans." It's reached the point where saying "where are your from" can be synonymous with "you clearly aren't American, so you must be from another county." This is a direct result from true racists being ignorant.

Is that implication always the case? Of course not. Should people always get offended by the phrase? Of course not. But enough bad apples of ignorance has turned the phrase into something implicitly rude. So in order to avoid implying "you think I'm not American?" Proper phrasing prevents any misunderstanding.

In the case of this thread image, the women sounded like she assumed the other character couldn't be American, solely because of his ethnicity. Did she mean that? No. Did it sound like that? Yes. And sounding like your implying someone can't be American because of their ethnicity, is offensive.

And again, this isn't because of sensitivity issues (though it does play a factor which will probably dwindle out in the future). The problem is historical racism. The phrase has been used in an inappropriate way long enough, that it can be misinterpreted as ignorance.