Religion -- at least the Abrahamic kinds -- is an exercise in cognitive dissonance. One has to believe two contradictory premises at the same time. The monotheistic, patriarchal religions of the Mideast are based on the Stone Age legends of people who had no idea how the universe worked. The more we learn about the universe, the more it conflicts with Abrahamic tradition. So the patriarchal monotheists have two choices: back off from the more pathetically ridiculous tenets of their faith and strip it down to something like Unitarianism, or hang on to the drenn and try to discredit anyone who challenges it.
For a long time it seemed that the Unitarian faction was winning out. Not that the actual Unitarian Church was swelling in membership, but most of the mainline Protestants, a good many Catholics, most Jews, and a growing number of Muslims were coming around to the same conclusions as secularists, that Santa Claus and the Easter Bunny and Jack-o-Lanterns (and the equivalent Jewish and Islamic symbols) are good harmless legends to teach our children and that humans have within them, especially collectively, the power to create a good and honorable world.
But somewhere toward the end of the 1970s, the great intellectual movement that had gripped America since WWII lost steam. It was suddenly not only OK but downright attractive to be stupid and ignorant. The fundamentalist churches that had been pushed back into the fetid swamps of the Old South began to gain members. People with college degrees and houses full of man-made, technological miracles started feeling guilty over their wild hippie youths, yet felt that simply growing up and acting like responsible adults was too difficult a penance to pay. So they took the easy way out and joined churches that promised them forgiveness and in return only asked for a few hours of their time, an affordable chunk of their salary, and a promise to renounce reason, the scientific method, and all of the most important things they had learned about getting along in the universe.
What they didn't remind their "born again" Neolithic members of was the fact that they would have to embrace the cognitive dissonance of their ancestors. This had become much harder than it was in the era of the Scopes Monkey Trial. Everywhere they went they were surrounded by people who still knew that belief in supernatural salvation was not the best way to create a better world. So to avoid having to dialog with us, they fell back on what the Abrahamists do best: open conflict. We were no longer their friends, family, neighbors, and co-workers who disagreed with them over a philosophical point. If we were that, we might have to be taken seriously, or at the very least treated with respect. No, we became "Satanists."
"Humanist" had been a good enough label for years, but the concept of having faith in human nature no longer evoked feelings of disapproval in most people, so they had to come up with something stronger. "Satanist" it was.
You don't have to respond to the arguments of a Satanist. By definition, Satanists have the ability to dazzle even the most brilliant, well educated humans (i.e. those with an unthinking belief in the Abrahamic legends) with their arguments, arguments which by definition are evil and wrong. You can just dismiss Satanists without even listening to or reading their words. Better than that, you are actually under an obligation to NOT listen to or read their words, because you are a weak, sniveling piece of dog doodoo on God's lawn and you could easily be swayed by the persuasive arguments of the Satanists. No, the best thing to do with Satanists is ostracize them, if you have the bad luck to live in a country that doesn't allow you to shoot them or at least cram them into ghettoes.
So the answer to your question is: They call us Satanists because that way they never have to take anything we say seriously, and their fragile belief system based on coginitive dissonance can survive for a few more generations, either until the Abrahamists blow each other (and all the rest of us) up in their virtually endless wars and genocides, or until scholarship and education once again find a big audience and the Abrahamic churches sink back into the dark, backward margins of civilization, hopefully this time for good.
