Well that's because they have a metaphysical framework. Satanists merely allude to one while not actually believing in it.
Not really. Many of them don't even believe in the literal version of reincarnation that we have in the West.
Many self-proclaimed Christians don't actually believe in it either.
Again, nobody can judge whether something is a religion or not. If a person says they have a religious belief in x, then no one has a place to tell that person it's not a religious belief. Least of all the government, especially since we have the first amendment.