This is not even close to a good, sound or technical reasonWell, "good" is a little subjective. I am purely speculating here, but I doubt that Apple compiled something into the code to make it simply not run on first gen hardware. IE: there must be something about that hardware platform that makes it different enough from current-gen hardware that supporting it adds some amount of back-end effort. Perhaps the reason has to do with one of the other factors I mentioned, a "mistake" or shortcoming in the first-gen hardware that requires more internal hacks and work-arounds in the code than they are willing to support.
It's not like this sort of thing is unique to Apple. Some of the first-gen Droid phones are not going to see the 2.2 update. Yes, there may be more hardware variants and changes among the Droid phones, but it is still a relatively new platform abandoning not-that-old hardware.