If I'm not mistaken they don't expect every day users to learn that on account of metro apps automatically being suspended and then closed for you based on your usage.
The basic idea is that if you aren't using an app it receives a "suspend" event allowing it to save its state so that it can resume where it was. If the system is low on resources it will start terminating suspended apps that you have not used -- though as far as I gather they should still have the ability to re-launch and read in whatever they saved at the time they were suspended.
So the general idea is that closing apps isn't something you should be concerned about as a user.
EDIT: This describes a metro app's life cycle: http://msdn.microsoft.com/en-us/library/windows/apps/hh46492...
The basic idea is that if you aren't using an app it receives a "suspend" event allowing it to save its state so that it can resume where it was. If the system is low on resources it will start terminating suspended apps that you have not used -- though as far as I gather they should still have the ability to re-launch and read in whatever they saved at the time they were suspended.
So the general idea is that closing apps isn't something you should be concerned about as a user.