He didn't reply because he's afraid of losing voters who hold these insane views. As if he even needed them.
I thought there was hope for him when he publically said that the Birther movement was nonsense. But by staying quiet when some nut calls Mr. Obama a traitor, he's proving himself a weakling.
Oh, and I seem to remember Mr. Obama defending Mr. Romney at a speech when someone brought up Romney's Mormon background.