If you do not take action for people using the heavy ammo exploits you are basically telling anyone who can to use this exploit as much as they want since it will have no impact on them.
Bungie should come out with a public statement making it very clear that anyone using exploits to gain an unfair pvp advantage will be banned.
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They can't afford to ban people. They've lost enough players that's why they implemented the skill-based matchmaking.