The back matter is one more thing where you get input—usually. But again, that's been true only for the past decade. The purpose of the back cover is to entice readers to buy the book. In most instances, that is the work of the copyeditor or associate editor.
You usually receive the material for editing. My experience has been that the copyeditor grasped the content of the book and stresses that. The problem I've encountered is that it doesn't read the way I would write it. So I'm delighted I have a chance to edit it.