I asked because if you have gnome or kde with it you can configure the policykit authorization graphically using the gnome control center and kde system settings.
You can try using yast2 to configure who can shotdown and restart your system though if you are using gdm or kdm as your display manager. With xdm not sure about it, I have to be root to shutdown with it.
I had the same problem and found it was caused by installing the proprietory Nvidia driver. Replacing the Nvidia driver with the standard open source driver has solved the restart/shutdown problem for me.
Hopefully the issue with the proprietory driver can be sorted out.