I haven't heard them argue that they want to use "Atheism symbols" instead. They just don't want state-sponsored religious symbols, particularly when those symbols are Christian, because so many people believe that Christianity is our national religion which is bullshit. Also, American Atheists don't give a damn about stuff that is on private property, only stuff that is owned by the government on government soil. That is, in essence, state sponsored religion, which IMO is unconstitutional. IANAL though.