These Satanists are a little mixed up. The pentagram is a symbol warding off demons, at least in Goethe's "Faust."
Mephistopheles is trapped in Faust's study by a pentagram, which because it was poorly drawn, Mephistopheles didn't notice when he slipped in.
Then he can't leave, because of the pentagram: "Die Sache sieht jetzt anders aus; der Teufel kann nicht aus dem Haus."
The “upside-down” pentagram was the one associated with devil worship. That notion came from Eliphas Levi, apparently. The goat’s head appeared in the 1800s.