It's because Facebook thinks drag queens should be able to use stage names like "Lil' Miss Hot Mess," but doesn't think Christian Clergy should be allowed to use anything but their legal names.
See "Stories from the Culture War Trenches" by Rod Dreher.
Update: They have done the same thing to Fr. Tryphon, who if he used his legal name (which I don't even know) would not be recognized by anyone who hasn't known him since he was kid.
Facebook has gotten too big for its britches. It's time for the users they depend on to whittle them back down to size. Refuse to advertise on Facebook until this policy is reversed.