About physical copies of Windows 8 Pro

So my dad recently bought a couple of physical copies of Windows 8 Pro from Officeworks for something like AU$54. I just assumed from the price that it was a physical version of that $40 upgrade offer you can obtain through the Microsoft Website (until January 31st). Since the OEM versions (for people building their own systems) are upwards of $100, this seemed like a logical conclusion. The only thing is, when I looked at the packaging for the Windows 8 Pro 'upgrade' box, there was no small print to indicate that it was an upgrade copy and that you needed a previous installation/legitimate serial of Windows XP, Vista or 7. That seemed like a really stupid omission that would cause a lot of consumers some headaches, were it actually an upgrade-only installation.

Later that week I asked Dad if he'd tried installing Windows 8 Pro onto a computer yet. He said he had, and it was a clean installation on a newly built computer -- surprisingly enough, there were no issues. I asked if he had to put in the serial of an old copy of Windows 7 or anything. He told me the installation didn't ask for a serial apart from the one that came with the Windows 8 Pro box. He did this a few more times with different systems and different copies, and they all worked fine too. I want to know if I'm missing something about the installation, or if anyone else has tried this, and whether this is supposed to happen? Because it certainly saves a lot of money compared to getting an OEM version installation for more money (not to mention without a physical disc either).