What nationality are you?

I took them for a ride on the 105 line for almost two tears. I didn't mind it, it's not as bad as a lot of drivers make it out to be. Still, it has it's moments. This incident happened not long after I came to America so perhaps that's why I didn't understand the question.

One evening I pulled up at the traffic lights at Vernon and Central. A gang of kids just appeared out of nowhere. I had to admire it really, the whole thing was done with almost military precision. The lights were red, out jumped the kids armed with spray paint, before the lights could turn green the windshield was covered in red paint. Then they were gone.

I called up the control room and told the dispatcher that I couldn't drive the bus any more on account of how I couldn't see a thing. I described what happened and then he asked me the strangest damn question. He asked;
"What nationality were the kids?"
I replied that as far as I knew they were all bloody Americans. It turned out that what he was really asking was, what was the color of their skin. I didn't understand that at the time and to tell the truth I still don't. When I've mentioned this to any one who is American they understand what he was asking but it gives me a question;
If this is supposed to be "One nation" and if we are supposed to take a pledge of allegiance to this "One Nation under God" How can you ask anyone here "What nationality are you?"
I still don't understand after nearly 19 years so could someone please explain.