Atheists believe in science and what is proven, you must accept that. Atheist monuments would tell people to trust science and to get educated, which is obviously a good thing. We need science to progress, so we need atheist monuments. Physically saying, we should bank religious monuments, unless they did something important that does not support religion.
Most of the early states in the United States were founded on religious principles. Our country was started with freedom of religion in mind, not necessarily freedom from religion. While individual groups can certainly put up athiest monuments if they wish to and get permission to, it would be unusual for the United States to endorse and support these monuments, because it would be like endorsing a specific religion, which the government is not supposed to do.
Monuments of all kinds in the United States should be secular. Those monuments erected by the government should respect the first amendment's prohibition of passing laws respecting religion. By the same token, if a private atheist group wants to erect an atheist monument on private property, they should be entitled to do so.