Most of the times we consider accessibility of only the published content. Not of the interface where the content is created or managed. Most popular reason being that the published content is where the meat is, the administration interface of content management interface or the backend is only the means, not the output itself. And that the userbase of content management can be controlled. However, in this user generated content era, we are seeing the user has entered the administration interface.
A good article to read and understand what to look for in an accessible CMS.