You have a point, just because the mini is marketed as a budget machine and the displays are really meant for the pro line of computers. However, since Apple has no mid-range headless Mac, people who want both an Apple computer and Apple display have little choice. They can either get a Mac mini or a PowerMac G5--and for most people, even if they have $1000-$1300 to spend, that choice will be the Mac mini. In theory, Apple wants these people to buy the iMac G5, but if you want your computer and display separate so you can upgrade them separately, you're likely to go the cheaper route.
However, for people who really are buying the mini just because that's all they can afford, the Apple displays make no sense.