There are certain pages, like the "Your account has been restricted" page and the login page, that are indexed. It also makes little logical sense to index the search page and the notifications and chat page. And it is also pretty standard to create a robots.txt file on any website. Otherwise, a bunch of bots like Internet Archive bot will copy pages that are not meant to be downloaded. Not that it is a bad thing, but it can potentially slow the site down for everyone.