The chip version of the BU-353S-S4 is PL2303HXD. The note that they have included at the end states that they will be supporting the following chip versions once the Windows 8 OS has been completed.
As the Windows 8 OS currently in its testing stage, Prolific is unable to provide a complete functioning driver. The ones provided in the website are considered test drivers.
In other words, since there is no official Windows 8 version available, Prolific is unable release a proper driver unless an official version is released.
Thank you for understanding.