This is my theory as to why Genlocks can use magic and dwarves cannot, The overuse of magic by the dwarves in the Primevil Thaig and exposure to lyrium has rendered all future generations of dwarves unable to use magic but not entirely. Dwarves still have the ability to use magic but its a recessive one. When genlocks are born the darkspawn corruption makes the recessive magic gene a dominant one thus somtimes creating genlock emmisarys.