Actually, I run a phpBB forum of my own. And I do restrict viewing certain information to registered users and even classify stuff based on user profile (who can view what stuff).
First, one idea behind it is that there are certain things that you want to be publically viewable and, therefore, exposed to search engines and stuff. Meanwhile, registered users can enjoy a certain security of not having stuff that they post necessisarily broadcast to the 'net in general. Of course, on a set of forums deliberately designed as a public source of information for software QA, doesn't make sense to hide EVERYTHING.
Secondly, there are web-bots out there that specifically target phpBB forums (and other forum types, as well) that, if you have stuff exposed out to non-registered users, the web bots can SPAM your forums with all sorts of large lumps of pink canned meat. So, you secure your forum so the bots don't touch it.
But, even this can be mitigated by restricting starting new threads and posting replies to forums to registered users but still allow non-registered users to read the stuff.
What this looks like is a rather amature attempt to create an online SQL forum without really knowing how to use the forum code that they selected.