Those who aren't posting aren't getting paid, so it's fair to them and the business owner.
How is that fair towards the business owner? Yes you pay less, if people are not posting, but you miss out on the potential advertisement space, provided by people who post their full quota. {If I only post 5 times in a month, and someone are more active and they can post 50, as a business owner, looking for max value for money, would go for the person, who post their minimum 50 post, which increase my companies exposure.} Your statement makes no sense, please explain to me, why you score more from people, who post less, than their minimum?
Only way that can be true, is if the income generated from this campaign, is less than the budget, you are paying towards funding this campaign. {Then the reasoning would be, why do it?}
If, my observation is wrong, please correct me. {My intent was not to argue, but to point out, that members with low activity, is hurting your exposure}
If you felt offended, I would be more than willing to apologize.