To my way of thinking, it's really not a question of ethics as much as a matter of practicalities.
1. It's easier to kill them when their wings are set and vitals are exposed.
2. You've got less chance shooting up the decoys when the ducks are in the air.
3. Live ducks make very realistic decoys.
4. If one needs motivation to pay attention, not shooting them in the decoys can serve as penance for not seeing them come in.
None of this should suggest shooting ducks in the decoys is wrong.
