As you have discovered, the upgrade version of Windows 8 relies on a previous Windows installation being on the machine it's installed on. All previous upgrade versions worked like that, this one is no different in that respect.
What is different with the Windows 8 upgrade is that you cannot subsequently install the previous version on a second machine.
Microsoft's rationale is that you've paid for a full licence for the earlier Windows version, and you're more likely to upgrade if you're only asked for a relatively small amount for the upgrade. You haven't paid for a license for a second installation of the previous version.
You can use your upgrade on two computers however, provided there is a properly licensed earlier version installed on each of them.