I find this post rather offensive, and as a Satanist (of the actual, devil-worshipping sort), I feel the need to defend our position.
Satanic philosophy and Thelema are extremely similar, or even exactly the same, depending on which Satanic group's theology you're going by. The connection is so strong that many of us are even Thelemites. The ideas behind Crowley's magical systems, unlike those of many Wiccan and neopagan ones, perfectly complement our philosophy and clearly come from the same Left-Hand Path foundation. While this doesn't make Crowley a Satanist as such, it DOES make him someone who believed and practiced exactly, or almost exactly, as we do, and so he is an important figure in our tradition.
Now, if there's a group out there claiming he was definitely a Satanist, I can tell you why they say this: Crowley wrote a lot of very Satanic things, from both a philosophical and aesthetic standpoint. For this reason - and that a lot of Crowley's spiritual experiences match our own - a lot of us feel that Crowley was a closet Satanist. He may very well have been, because
people were still being imprisoned for witchcraft in Britain during his lifetime, and if he declared his Satanism openly rather than just using the symbols, he'd be in a world of trouble. This is certainly a possibility; however, we can't retroactively claim Crowley as a definite Satanist if he didn't call himself one.
That's basically why we Satanists have such a fondness for Aleister Crowley and some would argue he was a closet one himself.