The Leave page, leave.html, is in the groups context. It is necessary for this page to be outside the group context, otherwise the former member would get a Permission Denied after he or she left a secret group.
Rather than simply providing a button that removes the member from the group, the Leave page presents the user a choice for reducing the amount of email. The hypothesis is that members often leave because they get overwhelmed by email. Giving the member the choice of going onto Topics digest or Web only may reduce the numbers leaving.