GNOME Shell seems to turn off all ways of locking the screen when using e.g. KDM or lightdm to log in.

I realize that GDM might be the only option that can actually lock GNOME securely, without someone else being able to kill a screensaver application and then take over the session.

However, refusing to lock at all when not using GDM seems like an odd choice that only makes those computers less secure/private.