It's correctly named IF we understand that the exorcist who actually expels the demon is not the official exorcist, Merrin, but rather Karras, who expels the demon not by the Roman Ritual, but through his own courageous, altruistic, Christlike sacrifice for Regan. The Ritual frustrated the demon, but Karras was the one who took the demon out the window with him. Thus making Karras the Exorcist in the truest sense.