It's vital for the forum to be closed for non-members and search engines etc. There is now a policy in place which prevents using the forum with a nickname or a pseudonym or whatever. It's Real Name Only. To make that policy work it has to be closed.
In other words the forum is much more strict, matter-of-fact, no-nonsense and "official" compared to this forum which is basically crazy. It's good this way. The craziness can stay here and the foundation business can stay over there.
Being read-only for non-members would be good for openness and transparency. I can clearly see the benefit of non-members not being able to post to cut out a lot of the rubbish you see on this forum for example but I don't see any benefit to denying non-members read access.
As Technomage pointed out, the problem with opening the BF forum to the public is that (mostly) real names are used. It should remain closed to search engines and general public. We can and will continue to discuss important issues here, on this forum, and BF will not change it whatsoever. I am sure any important issues pertaining to Bitcoin which are brought up in the closed BF forum will find their way to this forum too. I'll do my best anyway.