First question, why are there boards where the signatures cannot be displayed or aren't visible on the forum?
Those are sections that are meant for serious discussion, people posting in sections like that are doing that because they want to make contribution to to post, or let me say they really want to pass information, and they are not doing it to fill their signature post, you wont see people spamming in sections like that because posts in those sections wont be counted.
Second question is why do campaign managers exclude certain boards for posting and others where posting is allowed?
What's the purpose of wearing signature if you are posting in a section where the signature and Avatar wont display? If you are posting in sections like that then you are not marketing any project because your signature is not displaying, and if your signature is not displaying then what's the purpose of the project paying you.