I really get annoyed by the "Freedom From Religion" group. They whine continuously about non-issues like a Post Office displaying a sign like that. There's a pretty big difference between something like that and actual state sanctioned religious preference, but to hear them complain you'd think people were being force marched into churches every Sunday at gunpoint.
And they lecture us not to whine over molehills.
All of it is based on a complete misinterpretation of the First Amendment. The First Amendment was meant to ensure that there would be no state sponsored religion, which protects other religions from being outlawed. It was never intended as a mechanism to force religion underground or to sanitize religion from all but the most private of places.
"He who takes offense when no offense is intended is a fool, and he who takes offense when offense is intended is a greater fool."
"Don't take refuge in the false security of consensus."