At this moment Microsoft only allows to buy an upgrade version of Windows 8 Pro.
So the legal way for customers who do not have any version of Windows installed will be - buy Windows 7, install it and upgrade to Windows 8.
We are waiting for the thing named "SYSTEM BUILDER FOR WINDOWS 8 PRO" which is intended to be used in virtual machines and when you are going to install Windows 8 on a clean PC.
Thanks,
Alex