The reasons that you don't want people to use spotlights for killing the animal are not too hard to come up with. (Actually kids on college campuses do this for zombies-vs-humans, smart zombies will blind humans at night with spotlights to make it hard for them to fire their nerf guns back at the zombies.)
It is considered unsporting/unethical for a couple reasons: first because the animals like to feed nocturnally so it's easier to find them near people's fields, so there's no “tracking it through the forest” step; second the animals will often freeze and stare into the light, making them easier to target; third you can't see behind the animal to ensure that the shot is safe if you miss; for all you know you might be aimed at a neighbor's house in the distance after they've turned out their lights; fourth, getting a spotlight in your windows is a mild nuisance while folks are wandering around near your property; fifth because nighttime hunting is in general discouraged or illegal because that's when the poachers would be out and when you can't see other humans very well.