Assuming a decent campaign manager earns more than the users in the campaign, that would be a dumb thing to do. I also think that if it would happen on a large scale, it would have been discovered once in a while. So it can't be happening a lot.
Greed can make people want more even when they have enough. If a manger is not content with his pay he might be tempted to enrol alt(s). Since the campaign management market is competitive, a campaign manager might want to charge lesser fees just to get a job. At that point he might start seeking for diverse means to increase his earning which night include enrolling his own accounts in the campaign. Just as others have stated reputable campaign mangers will not engage in such misbehavior because they are concerned about their brand/name. I also agree that this behaviour happens rarely.