So it is voluntary - voluntary on the part of the States.
That is why this case abuts the Establishment Clause. While individuals can opt-out, it is still the state making a statement that endorses monotheism.
You, my friend, have just illustrated why this case is not about the Establishment Clause.
The First Amendment doesn't prohibit the States from doing a darn thing!
By that standard, Brown v. Board of Education is unconstutional, too. Was it not the states that ran the schools in Kansas? Was it not the Supreme Court (the federal government) that intervened when Linda Brown's right to an equal educational opportunity was being denied by a state's policy? Is it not the Supreme Court that has ruled creches on public land (be it owned by a state, the feds, or a local government) unconstutional?