If you want to state where you are from, it is common to leave out the article "ein". For example, if you want to say that you are from Berlin, you say "Ich bin Berliner" rather than the sentence JFK used.
Yeah, I had always heard that he should have left out the "ein." The rest might be a myth. I didn't know that only applied to where you come from, though. In language classes, the lessons don't generally involve name-calling. XD