I am not sure if there is a rule about creating fake conversations by the same group for the sole purpose of bumping threads. But if not, that should be added. If an entire thread page is covered with posts of the same group of users and that same group dominates the entire thread with unnecessary comments, admins should react to it.
It is supposed to be considered as spam and all the conversations created by the shills should be deleted and I am also agrees that there should be a rule against such kind of behavior because bumping threads using incentives is against the rule so the thread will be immediately trashed but if they are using the account bumpers and paying out which is clearly visible but still there is no action.