They are 'giving it away for free', because this is basically Windows 8.2.
They need to move people off Windows 7, and also remove the taint of Windows 8. They are hoping to generate revenue via other avenues. The Store, for instance.
Not many people actually 'buy' the OS to upgrade. They get whatever OS comes with a new PC, and use that until it dies. It costs MS little to provide a 'free' upgrade from Win 8.1 or 7.