From your description, it's possible that the "all registered users" group in your instance has been granted admin permissions at either the system admin or global social group level.
To confirm, you can check the following pages in your admin console
- permissions >> system admin permissions >> check for "full access" permissions
- permissions >> social group permissions >> check for "manage groups"
For more information on permissions you might also find the following helpful:
Lastly, if you are still seeing this behavior after confirming that the correct permissions are in place, I would recommend submitting a case in your customer group here in the Jive Community, where a member of our support team can assist with further investigations specific to your instance.