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Is it ethical to capture a pokemon using a Master Ball?
From what I understand of the series lore, wild pokemon generally stop trainers and battle their pokemon because they naturally have a desire to fight and grow stronger more so than them defending territory or bloodlust. Capturing a pokemon by weakening it signifies to the newly caught pokemon that the trainer has already trained stronger pokemon than it or knows better battle tactics, either way, that the trainer is worthy of being its master. Even in the case of a trade or a gift, the trader/gifter still has a chance to evaluate the person on the other end and make a judgement call as to if they will train the pokemon they are given properly, and the pokemon can actually disobey the trainer if they don't have enough gym badges i.e. are not sutible for them yet.
Simply throwing a Master ball at a pokemon completely bypasses that critical first encounter. Theoretically anyone could get their hands on a Master Ball, a child who only has a lv.5 starter could capture a god with one. It just doesn't seem fair to the pokemon, and realistically I think they would be banned, or should be anyways.