Why the inconsistency in penalising tampering?

The South African cricketer Faf du Plessis wears trousers with zipper-lined pockets. The ICC does not favour such on-field attire and has decided to phase it out. Yet for reasons that have become the subject of much ado, du Plessis remains partial to them. Certainly these were the trousers in which he was clad during the Dubai Test, when he chose to scuff the ball up on said zipper. Television close-ups revealed him forcibly and repeatedly rubbing one side of the ball against the zipper's tab. His action has violated the rules of cricket and earned du Plessis a fine from the match referee. These are the facts, and they are not in dispute.
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