Users sometimes go to the Group Public instance unexpectedly, and that causes trouble or "social drama". This can be mitigated if the app does not select a Group Public instance automatically.
When you open the world detail page in-game, the app selects an instance as the default. I am unsure of the specification of how the app chooses the one, but it could be a Group Public instance.
Although it is explicitly shown as Group Public, and users will see the popup message "You have joined a Group Public instance" when joining, people actually go to Group Public unexpectedly or accidentally.