I'm speaking from a U.S. perspective, so it may be very different in other countries, but here we have quasi-religious cultural markers, such as the story of Santa Claus bringing presents at Christmas or the Easter bunny bringing eggs at Easter, which everyone agrees aren't real but which have symbolic/traditional/cultural value. There's virtually no anti-Santa Claus or anti-Easter bunny pressure by atheists and many join in those cultural traditions.
The problem is that many people who promote religious practices don't see them as cultural markers. They wouldn't want Jesus and the Bible lumped in the same category as Santa Claus and the Easter bunny.
I think it's that insistence on "realness" that makes some atheists uncomfortable. I don't think they ignore the fact that religious symbolism has a powerful cultural effect. On the contrary, that's what scares them. And the same may be true for members of minority religions, who may see entanglement of the majority religion with government as a way to promote the majority religion and eliminate their own.
People who want secularism, in the sense of a separation of religion and government, may feel that their own cultural traditions and beliefs are at risk if the majority religion is able to pass laws based on its version of reality, which excludes their own version of reality.
the above would be helpful to illustrate some of my concerns. hence, i endorse the above and makes it as a citation for the request of Mr. Richard Garfinkle that "You're repeating your earlier assertion. That's not evidence. Could you please instance such organizations and explain why you think they speak for the majority of atheists and secularists."