Reply #46: One valid reason is to give a breath of fresh air to atheists and other non-religious people.

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The sign is for people who don't believe in god, or who are agnostics, or who already have doubts about Christianity. The sign is "preaching to the choir", so to speak.

To a non-religious person, religious messages often get heavy handed at certain times, such as the two month Christmas season. Signs like these are a breath of fresh air, and reaffirm the American value of Freedom of/from Religion.