They are doing it because '09 requires Leopard, and a lot of the install base doesn't have Leopard. It still has Tiger, and Apple plans to drop security updates on Tiger in a couple months when Snow Leopard releases.
The upgrade momentum is slowing in Mac land, so they have to do things like this box set.
Now is the time to hit them with a demand for five years of security updates on the OS!
The upgrade momentum is slowing, so they should make it less of an incentive to upgrade?
It makes sense that they would...
It makes no sense they would. With the box set, it looks like they're trying to have an incentive for the 10.4 stragglers to upgrade. Plus, they get to clear out inventory of 10.5 install discs.
With 10.6 seeming like it's Intel only, keeping the 10.5 box set around when 10.6 comes means PPC users can upgrade to the latest software their hardware can run.
I wouldn't be surprised to see a 10.6 box set when the next iLife/iWork is out, shortly before 10.7 comes out.