No one said you have to be an active member with a minimum number of posts to promote your book. Just that there should be a place specifically for this purpose.
What I said is that
active members mentioning their book in several places around the forums are not annoying, because it is not usually meant as an advertisement but as part of an honest discussion. Not that they are the only ones allowed to do it
Why is it not a good idea to have all promoting in one subforum? Easy for the readers to find (or to avoid, if they wish). I'm sure we are all interested, we just don't want ads popping up in unexpected places, in the extent that it can be avoided.