Clerics have spells, but not every priest does. Spellcasters are rared and considered favored of God.

Arcane spellcasters are, naturally, burned at the stake. Except for those operating in secret cabals, as they try their best to hide their magic practices from the public.

Assuming all spellcasting classes are rarely encountered, would the 'no druids' rule be necessary? I mean, shapechangers fit into the Middle Ages myths more than spellcasting clerics of the Church do.