To me the ethical issue comes down to intentions. Sure the duck might not know the difference if you shoot it, dress it, and take it home to freeze for the winter, versus shooting it then seeing how many pieces you can make out of it with an M80 firecracker. But morality isn't really about the duck, it's about the humans. It's a concept of humanity, and we made it up to consider our place in the world, and what the point of our existance is. The way I see it, if I'm making paste of a bird for fun, then I don't have much place in the world, and I'd feel pretty damn worthless about myself. Whereas if I'm spending my energy getting food for the family, and being thankful that the duck provides that, then I just feel much more purposeful and just feel better about being here.
Same with reefing. I've wondered before whether this is a good thing to be doing, because it does have at least the potential to do damage to reefs, but it also helps us and others learn about them and appreciate them, teach our kids about them, etc. That makes sense to me, and seems purposeful. Starving a fish in a box of it's own crap doesn't give me a mother-earth kind of feeling though. Sure it might be the same to a dead fish one way or another, but morality really isn't about the fish, it's about the humans.