>>49014520Wild Slowpoke, like those found around Azalea Town, use their tails to fish for food. Removing the tail therefore dooms the Pokemon to starve until the tail regrows.
Harvesting Slowpoke tail doesn't cause the Pokemon harm, so doing so with a domesticated one that doesn't use its tail to fish would be no more unethical than milking Miltank for its Milk, Taking Chansey's Egg or plucking the fruit from Tropius' neck. That's the difference. TR were taking the tails from wild Slowpoke and leaving them to starve.
