As per basic rights and freedom of speech, we all should be allowed to write anywhere of this forum (as long as this forum allows) and a campaign may exclude posts as per their rules from counting toward payment and should not cost the spot if minimum post count is reached after excluding posts which are done on excluded section/board/topics.
Oh, the freedom of speech! Oh, our rights are being violated!!!
You have the freedom for writing everywhere you want, that doesn't mean you have the right to get paid!
If they pay for let's say 50 posts and you make 10 posts in the boards they pay for and 40 in off-topic the campaign manager might think he has a better alternative, a poster who posts in the areas where he needs to maximize his advertisement and since they have probably limited slots it doesn't sound right for them to keep someone who is not adding as much as others to the pile. Just because you reached the minimum requirement it doesn't mean you have some legal contract and the manager must preserve your place for eternity.