"Atheists" aren't satanists. Problem is to believe in Satan you have to believe in God, and vice versa. Satanists who CLAIM to be atheist are, ipso facto, not atheists. On the other hand most "Satanists" are people kneejerking towards the open bigotry by the "religious" people. I don't think many, if any of them, actually "worship" Satan. They simply see him as a symbol of freedom, of ever questioning authority.
No, most satanists don't worship an actual deity named Satan.! They tend to absorb a fair share of mystical mumbo jumbo from what I've seen. When I considered atheism as a cause to promote and was a member of a local atheist meetup group that we were trying to build into a full fledged social club, we had a couple of satanists who were part of the club...but did have their quirks....psychic energy forces, existence of spirits or souls, ....they weren't the type to burn down churches of course, but they weren't strict metaphysical materialists either...so they didn't completely fit in. But we did recognize that they fit the basic definition of atheist nevertheless.