I think it just pays Apple more in customer satisfaction to have everyone on the same boat. Simple economics. They make the money on the hardware after all. Same thing for free iLife and iWork, customers pay for the complete experience in the price of the device.
It also helps developers. It would be a reasonable position to take to only support the latest MacOS release and features. (People who don't want to upgrade their MacOS version for free are also less likely to want to try your software or upgrade to new versions of it.)
