Guest can Post on Groups Timeline

I've found that when I added groups and acting as a guest, visit one of the groups the click on "Timeline" last link.

You will see the post to timeline. It posts as "former builder" as I changed the  "guest" key. 

Is this a setting or a bug? 

I went to the View Group Submenu and made that link only visible for site members to keep them from making a post for now.