Acceptance.

For every reasonable belief system to be accepted without censorship, as a people, the government can not endorse any belief system or their icons. What of our Muslim, Hindu, Buddhist, and atheist citizens, etc? Do we also put up Muslim Koran scriptures on stones at Mt. Rushmore? Do we build Hindu statues in front of our town halls? Do we have a 30' budda sitting and laughing on the White House lawn? Do we put "God is Dead" on our coins? Do we build a Wiccan alter at Arlington? Hell No! For all of us to be religiously free, we must keep our government from choosing a religion or using relevant religious icons. (I believe using religious icons of dead religions, from Greek and Roman mythology, in the tradition of western culture is acceptable since they don't invoke any practiced religion)
And about prayer in school: Again, what of our atheist citizens? A moment of silence does impose a religious practice on to their children.