I think it should be mandatory because it increases safety for cyclists. It could be enforced with a $100-300 fine.
Fines in Denmark are $121 (DKK 700) if lights are not used. As someone here on another thread suggested, the fine could be a 50% coupon that the rider uses to purchase a light. If permanent lights are compulsory the prices of lights would probably go down as well. Making running lights a requirement during the daytime would also decrease the problem of ninja riders at night.