Given the incredible amount of boosting in Rumble, and the nature of the game mode itself, I really do not understand why people are allowed to enter Rumble games as a fireteam.
But if Bungie is hell bent on keeping it the way it is right now, they should at least make it as such, that kills made in Rumble don't count towards the completion of said bounties, if the Rumble play list has been entered as a Fireteam.
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Or Maybe your friends want to see who is the best out of each other and want to have fun. It's not always about cheating the game.