People ask me that question all the time. If I simply answered "American", people would either think I was offended by their query or being a smart ass.
Why do you ask a loaded question and then get upset when people respond to what they think you're asking? Seems to me you're getting mad because it didn't follow the pattern, "ask a stupid question, get a stupid answer." If you need something to be upset about, I guess that's as good as anything else.