Currently, this setting only does one thing:
If a space with a viewport has restricted the access to the Confluence UI, a logged-in user who is not allowed to see the Confluence UI will be redirected to a "not permitted" page when trying to access a page in that space.
This doesn't really make sense, as anonymous users are redirected to the viewport in the same scenario. The logged-in user should have the same behaviour, which makes this setting obsolete.
Also, anonymous users should always be redirected to the viewport view.