I'm not saying its justified. I'm just giving you the reason. Also you can't compare iOS and OS X update paths.
Uhm, nope. Not "selling" something currently has nothing to do with not allowing it to be updated, even if the hardware supports the update.
The reason is that Apple thinks that by withholding the upgrade, users will rush to update their suddenly "old" iPads when the Mini is announced. This "we don't sell it, so no support" policy is the reason I will not buy another Samsung-branded phone, and now Apple is doing it as well.
And why exactly can't we compare iOS and OS X upgrade paths, as long as the hardware supports the upgrades? This assertion makes no sense.