Why is it so offensive? Why not just let the speaker know they weren't American and move on? The instant escalation to violence seems like part of the problem

Why is it so offensive? Why not just let the speaker know they weren't American and move on? The instant escalation to violence seems like part of the problem generally in today's society, which extends to non racial topics like politics, gender fluidity, etc. A more appropriate reponse would be something like, "Why do you think I'm American?" A simple question like that would likely be sufficient to get the original speaker to think abkut and reorient their world view, and there was good-faith discussion the entire way.