Simple answer - the Private Forum is a free resource for sensitive business topics and discussion - not a place to get commercially advantageous information for free in private.
I'm happy to give free SEO advice in public for the benefit of all members and guests - and hope that guests reading useful stuff will be encouraged to join.
We also had a situation in the early days when there were more questions being asked privately than publicly which makes the Forums look dead to guests and search engines.
At the end of the day this is a free public Forum with the only free private area that I'm aware of - so I think I'm entitled to make the rules on it's use