This is not exactly an issue related to kubuntu.
You chose KDM as default display manager at installation, GNOME however depends on GDM in order to do root-permission-requiring operations like reboot and shutdown. The very same applies for KDE, just that it depends on KDM for these actions.
Eventually you could report a bug to provide these features on a generic level independent of KDM/GDM or implemented in a display manager agnostic way.
This is not exactly an issue related to kubuntu. -requiring operations like reboot and shutdown. The very same applies for KDE, just that it depends on KDM for these actions.
You chose KDM as default display manager at installation, GNOME however depends on GDM in order to do root-permission
Eventually you could report a bug to provide these features on a generic level independent of KDM/GDM or implemented in a display manager agnostic way.