Because that argument is way too subtle for most people, and people want to give him the benefit of the doubt.
To me, reciting the call to prayer in Arabic, and knowing what it means, is like a baptism. He is Muslim.
I don't agree with this...he could have rejected Islam in favor of Christianity. If I speak and understand French does this make me French? There are plenty of other reasons not to vote for him and I believe that softness on terror is one of them. Another reason, of many, is the black version of Christianity that he's been listening to. This guy is not the great unifier that his supporters think he is.