They don't to support older OSs anymore, maybe Leopard could have been in there, but I think they want to move away from that as fast as they can, especially with Lion on the horizon.
And SL is still available for 29 USD for all Intel Macs.
It makes it easier for developers; 10.6 had a lot of new features that were _very_ useful for developers, but only if you don't support earlier versions. This move makes it a lot easier for developers; anyone downloading your app from the app store is running 10.6.6.
And experience shows that people who don't want to pay $29 to upgrade from Leopard to Snow Leopard, and even more people who are still stuck on 10.4, aren't the people who are most likely to buy new software anyway.
They probably are just concerned about the compatibility between Apps and PowerPC since 10.4/10.5 can run on a PPC. So even though you run 10.5 on Intel, there are many people who don't, and that would cause issues.