As I have experienced it, the Sikh turban reflects a form of difference that can provoke some Americans to react quite viscerally. Yes, ignorance plays a part and probably amplifies that reaction. But it may also be that visible marks of religious difference like the Sikh turban are lightning rods for this hostility in ways that don’t depend on accurate recognition.
I am not sure why the reaction can be so visceral — perhaps because wearing a turban is at once so intimately personal and so public? Walking around Philadelphia waving, say, an Iranian flag probably wouldn’t provoke quite the same reaction. A flag is abstract — a turban, as something worn on the body, is much more concrete and it therefore poses a more palpable symbol for angry young men looking for someone to target. Whether or not that target was actually the “right one” was beside the point for the Oak Creek shooter.
I am by no means suggesting Sikhs not wear turbans to avoid hostility. But I also don’t think we should fool ourselves that all hostility will be resolved purely by education, nor should we presume that this shooter suffered only from ignorance. As a white supremacist, it seems safe to suppose, what mattered to the shooter was that he hated difference — and saw, in the Sikh gurdwara at Oak Creek, a target for that hatred.