Because the Hawaii Department of Public Health has officially confirmed it.
You do realize that he is considered their "native son" and that Hawaii is majority Democrat and has been since statehood? That's almost like asking Mayor Daley's office for information on Mr. Obama and taking whatever they say as Gospel. But you are bound & determined to believe them, because it would be "racist" and un-PC to believe that a Chicago machine pol/community organizer could pull a fast one on the American people, with the help of communists, ACORN, LaRaza, the Black Panthers, unions and assorted other goons and thugs. Birds of a feather, and all that. But you keep calling us whacked-out "birthers" and we'll keep collecting evidence that this guy is not who he says he is, and is probably not even a citizen, much less a natural born citizen! When the rioters break down your door, you show them these threads to prove that you were on their side from jump street.