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?
Because the admin and moderators make the boards or thread not to contain signature.
Campaign manager exclude some boards because it is full of poor poster or full of what they do not want, like the off-topic. Another is altcoin board which bitcoin mixer will not allow because bitcoin is not an altcoin. Campaign managers have different reasons. You can see a campaign manager that will accept certain boards, while the other campaign manager will not.
Campaign managers have different rules actually, some will pay if you post in local boards, while others will not.