Most likely because the hardware of older devices will not be up to running a lot of the new features, or at least in conjunction with the existing features. I don't think it's Apple's intention to discriminate, rather to stop people complaining their iPad "now runs so slowly with iOS6 installed."
If you look at ANY peice of software you can purchase it has a minimum hardware requirement...if it won't run on your machine you either don't buy it or you upgrade you machine. The iOS operating system is just the same, except Apple know exactly the hardware you will be installing it on so can make these judgements to stop your device running slowly.
I understand your frustration, but like I say I don't think it's Apple discriminating!
As a user of apple devices it is my right to get the new features.
Sorry to seem harsh, but that's the most ridiculous assertion I've ever heard. You have no "right" to anything other than what Apple promised you when you purchased your iPad, and at no time did they ever promise you that your model would support all future versions of iOS and all the features therein. No, owning an iPhone 4 and iPad 2 doesn't make you a "bad person". But that also doesn't entitle you to all future upgrades and features.
Your iPad 2 does exactly what it did when you bought it, and will do more when iOS 6 comes out. That it won't do everything a newer model will do is just a fact of life, and applies to any new product. That's the entire point of companies coming out with new products, to do more than the old product did. Many products in the price class of an iPad don't every offer new features at all for an existing device, but rather force you to buy a new model for any new capabilities.
It's up to you what you wish to do. If you're that unhappy, sell your current devices and buy something else. But all manufacturers do exactly the same thing, so be prepared for disappointment whatever you end up purchasing.
It is better for the manufacturer to limit the release of new software to newer devices than it is to allow new software to run on older devices where suboptimal performance, crashes, and incompatible new features will degrade the user experience. The only exception I have seen with this is with some flavors of Linux, and even this holds true to Linux over time.