LeVayan satanism is not quite atheism, it's indifference. It is consider atheistic satanism because that's as close to atheism in Satanism as it gets (besides Rational Satanism which I'm still not too clear on). LeVayan Satanism is not necessarily about being a god in the sense of being a "divine" force, but more about being in charge of your own destiny as someone else put it (just to clarify).
Spiritual Satanism does not always include demons, devils, or even Satan. Satanism is an umbrella term that includes religions that have everything to do with demons and religions that have nothing to do with demons. The term "Satanism" does not only apply to anti-Christian or polar opposite Christian beliefs (devils and such).
Luciferianism is the best example I can think of off the top of my head. This religion is a type of spiritual Satanism that more or less associates with older ideas behind Lucifer than the Christianized "King of Hell". They don't really work with demons to my knowledge and I don't see "devils" in particular having much of a place in this religion.
Just shedding some light since Satanism is very misunderstood. I commend you BoneWalker for seeking to understand it better.