I doubt the reason is for the spam, or at least it shouldn't be a valid one.
B&H is an okay section, I occasionally browse that section and answer questions which I'm interested in or if they're asking for help about something. The real spammer's cesspool is actually Bitcoin Discussion.
No seriously, go look up a thread with at least 2 pages. You'll realize certain contents being paraphrased over and over again, worse still, some of them are outright incorrect. Unfortunately, you can't really ban Bitcoin Discussion from your signature campaign, the spam would just move elsewhere and that is where the Bitcoiners are always gathered. I do get some meaningful discussion from time to time but it's quite rare.
Agree on this one!
But there are still some meaningful threads specially newer ones which are really worth to look or respond on Bitcoin discussion board.
On the question on why beginners help is excluded into signature campaign valid boards is on where i do think that majority of only people
who would go to that board is only noobs.
So when it comes to marketing effeciency or effectiveness then they would definitely come after into those boards on where lots had
been lurking out.