Private member forums don't generate publicly available content or advertise the forum (since they're private).
It would also be hard to advertise private member clubs if no one can see them - someone might be interested in joining but if they cannot see the content there's not much to advertise the club other than the name.
Also, even if a topic is restricted to members of the club, it might have broader interest and be informative to people outside the club (it could trigger normal Pofo discussion if seen outside).
There may be a chilling effect on posting to private member clubs if the content isn't even visible outside the club. (which may not be very lively)
So, maybe these clubs (well...only one so far, but going forward...) would benefit if at least some threads would be "read-only" to anyone, and writeable to members? That would generate advertisable forum/club content while keeping the content within the bounds of the club.