As an atheist, this kind of obnoxious behavior really annoys me. It's one of the reasons many people hate atheists, and why many atheists are relucant to identify themselves as such. The organization behind it is called the Freedom from Religion Foundation. According to their co-president, Dan Barker, they put up the placard because:
No, it isn't important. It's stupid and counterproductive. Atheism is not a religion; it is an absence of belief. Why is a group of atheists acting like a religious organization and competing to have its own Christmas display? Holiday trees, Christmas scenes, and other religious holiday displays are completely harmless, and appreciated by the vast majority of the country. Although Christmas started as a religious holiday, it is also a secular tradition in much of the country. When a minority of atheists behave like a proselytizing religion, especially over something so trivial, it creates animosity toward the rest of us, and it accomplishes nothing.
it was important for atheists to offer their viewpoint alongside the overtly religious Nativity scene and Christmas-style holiday tree.