So my glib response: Not sure how successful that whole lock thing has been. Security Verification
My highly scientific response 
Okay. So beyond simply the frame of mind to going to a “protected area”, there is the following:
- The lock disappears after you select the post when when posting. There is no indicator that it is a private group at this point.
- When navigating in and out of posts in the latest section of the page, with the private and non-private content intermixed, people lose mental context to the category the post is actually inside. (Yes, I know there is a reference at the top of the page) They actually get freaked out thinking the private content is available to anyone before they realize that oh no, there is actually a lock symbol up at the top… but then they ask ‘locked for who?’, but it does not matter at that point because their trust is the system has been shaken. They simply opt not to post.