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, like the off-topic. Another is altcoin board which bitcoin mixer will not allow because bitcoin is not altcoin. Campaign managers have different reasons.