If you set forums viewable without being readable (view the content of a post) forums seems enpty (no preview are showed) whereas there are content in. Therefore, if all your community is readable only for guest, visitor will understand that there is nobody on the community. This is wasn't the case in 3.4 (we had "forum protected" option also). Something need to be done about it because it change totally the appearance of a forum and we can't really use this feature to protect some content as it let the forum/sub forum blank (bad appearence) and users can't really understand what it means.

I went into the language settings and changed 'there doesn't seem to be anything here' into 'Please login to see posts' - it does the trick temporarily until a better fix comes along.

Are you asking why guest can't see the forums and get a message like this: The page you are trying to...but may be available if you sign in. 

All my permissions are set that guests can view forums but can't post.