I don't think anyone assumes that at all. A poster was decrying the fact that satanists were considered to be baddies as they rejected the teachings of Christ. However, that is the social mores. People get their children christened. No parent says, I'm going to get my child satanised. It is neither good nor bad but thinking makes it so. People in society do find satanist behaviour shocking, whether they are right to do so or wrong, or even ignorant.
It is not true that Judaism, Islam, Hinduism, Jedi, etcetera, are considered in our society to be 'satanic'. Only religious zealots would claim that.
People do associate the occult with the 'dark forces' whatever that may be, whether it's the Nazi Third Reich, Alistair Crowley, the Masons and other secret ancient Egytian-styled cults and 'satanic' is simply the generic term used for them. Trying to narrow the term down to fans of Anton LeFey, Jimmy Page, Marilyn Manson or Damien Echols is reductio ad absurdum. A logical fallacy.