It was pretty obvious that the excluded participants may have defaulted and if the CM's assessment says so, he's definitely reserved with the right to exclude these participants.
Besides, only the CM is reserved with the right to decide on which post is eligible for payment and which is ineligible for payment, so those that weren't paid may have definitely had some ineligible posts which couldn't add up to make up for the activities/posts (20) required for payment.
The terms of the manager's assessment has to be know clearly to the participants. If there are prohibited topics, they need to be clearly stated in the opening thread. Also when you purchase a users signature, you do not buy their posting rights, so they can still post where they want as long as they provide the required 20 or 30 posts for the campaign, what you do as a manager is count the eligible posts and pay based on that.
The manager does reserve the rights to exclude members, but this in fairness has to be done in a transparent manner. Participants shouldn't be on edge, wondering which rule they are unaware of that they may have broken. Clear communication of expectation is very important.