To answer your question (and for that matter mine too), I bought a new Mac Mini just after the Leopard release.The Leopard DVD labeled as a 'drop-in' upgrade requires the previous 10.4 install. That disk was included in the box with a new Mac during the change-over time after Leopard was introduced.
Where did you get that installer disk? The commercial 10.5 install does not have that requirement. Otherwise, it could be tricky if you choose to erase the hard drive before installing Leopard, eh?
I disagree with your premise. Simple upgrade has been the standard method of installing new Mac operating systems since scratching in the dirt with sticks. It is the simplest and most reliable method of updating your OS. This why it is the default. For those who have problems, Apple also offers the Archive & Install, which requires that the user manually restore his/her settings, the fresh installation, which requires that the user redo everything.... why has Apple insisted that Leopard be installed (at least initially) over a version of Tiger?
Given that this can lead to problems, I wonder what motivated Apple to come up with this install method.