First question, why are there boards where the signatures cannot be displayed or aren't visible on the forum? Second question is why do campaign managers exclude certain boards for posting and others where posting is allowed?
I think the first question has been answered by most people here, so I don't need to answer it.
With regards to the 2nd question, I guess the best answer is that there are some boards that are either there are only few people who are going, and interacting into that particular board, or the signature isn't being displayed there. There are some campaigns that allows users to post on gambling boards because that board is where gamblers are most of the time therefore, it will benefit the company. There are some companies who prefer the participants to post outside gambling boards because it's not their niche.
Well, you can still post in those boards still, but your post will not be counted by the manager of the campaign.