Define a “real religion.” I think you will find it difficult.
In fact, that’s the basic problem with the concept of freedom of religion, in my opinion. There is no generally agreed-upon definition of religion in the first place.
With Satanism, we’re talking about several different possible things. There’s the “Church of Satan,” which is really just a form of atheism. There are confused teenagers. And there are stories, mostly told by Christians (some of whom claim to be former Satanists but often seem to be rather unreliable character), of horrific ritual abuse and sacrifice. Obviously the latter should be illegal–but specific instances seem hard to document.