They weren't really private, they just were not open to unregistered visitors. Anyone who bothered to register could see everything. I didn't like the impression it gave - that there were so many private forums. I wouldn't know, as someone coming here for the first time, whether registering would get me into those or not. I couldn't change the word "private" to something better like, "registered users only" in that forum view, so now they're all open.
That change was made about a week before the usergroup changes that this thread is talking about though.
And all the changes are being made, since we're on the subject, in order to prepare to abandon this forum software.