Oh stop it was just a joke.
Why joke about this?
To drive home the message that while Barack Obama was technically not born a hateable furriner, he might as well be for all that the Republican electorate, and thus Romney, cares. "Nobody doubts my Americanness - I'm a clean whitebread Protestant Ur-American! (Apart from actually being a Mexican Mormon, of course.) Now Mr Obama, on the other hand... he's a Black, you see, and a Black Exotic at that. He's so Unamerican we needed to see his birth certificate."