I think there is a possible conflict of interest, and therefore campaign managers should not be merit sources.
I don't see how that would be a conflict of interest: the campaign manager doesn't get anything out of meriting those posts.
I once read all posts made by ChipMixer participants in 2 weeks, I sent somewhere around 200 Merit to those posts. If a campaign manager takes the time to actually check the posts, it's very little additional effort to also Merit the posts that are worth it.
Why not bounty managers?
Can you show me a bounty campaign that doesn't encourage spam? I don't think it exists, because a campaign won't get any exposure on the spam-infested altcoin boards if it doesn't join the spammers.