That's because they will feel safer if he was a Muslim
Because if he is part of Organised Terrorism, then they can look to a strong leader (President Giluiani?) who will retaliate by being rude to some random raghead, which will deter future attacks.
But if he is an US raised nut, nothing can stop another US raised nut with totally diferent issues, making an attack at any time, anywhere.
I believe that's pretty much it.