Shutdown should be directly available from the desktop interface.
Simple answer, it isn't so you either use what's available or learn something new
or, find alternatives.A start button should also be there according to many of us but it isn't so you either
learn to live without it or, use a utility like
Classic Shell to bring it back.
If you choose the latter, you restore everything including "shut down" and, still have the "Metro Desktop" when you want to use it.