The impressive 18-55 2,8-4,0

Actually, I don't understand why you couldn't set the ring to, say, F5.6 and the computer in the camera would auto-adjust the aperture to maintain this F-setting as you zoom in and out. I understand the actual aperture would vary with the zoom to maintain a constant F-setting.

Obviously if an F-setting wasn't possible for a given focal length (e.g. F2.8 at 55mm) there would need to be an indication of that.

Can anyone explain why this is impossible or undesirable?