Atheists are against religions. It so happens that the dominant religion in the US is Christianity, so they seem to be against it more so than Islam. When push comes to shove, Atheists have far more fear and hatred of martial religions such as Islam than urbane ones such as Christianity, which as been de-fanged over the centuries. Atheists loathe Christianity, but they fear Islam.
"COEXIST" is atheist-speak for "Please kill me last."