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Because that one guy doesn't give you the Frustration TM if you haven't given your mon a bunch of herbs.
If there was another way to decrease friendship/happiness at a decent rate that would be one thing but they can't really add an option to actively abuse your Pokemon, and they can't get rid of the move now, not without blocking the mons that already have it from bank or something, which would be bit extreme just to fix this one minor issue.
