Wow that's a balanced source!
You had to register a religion while in Indonesia. So people who don't fit into any of the categories just put down Islam which is basically the default choice. Plus as Obama's father was raised Muslim, he was by strict Islamic standards a Muslim his whole life since Islam says you can't convert away, and his children should be raised as Muslim under such rules as well. So it's the easiest thing to put down here. Doesn't mean he was ever actually a practicing Muslim.
So...why can't he just say that to the American people?
Why lie instead?