We seem to forget that this is totally in line with the explanation that Apple has been feeding us all along: that their interpretation of GAAP dictates when they are allowed to provide updates for free. They amortized the money they received for the sale of the iPhone over a 24 month period, during which time they were allowed to add new features to the iPhone free of charge.
Now that it has been more than 24 months since the original iPhones went on sale, the 24-month amortization period for the original iPhones is used up. Which would mean that, if they wanted to add any more new features to the original iPhone, they would have to start charging for them.
But that would make average, non-savvy original iPhone owners angry, because they have to pay for something which is being given to other iPhone users for free. (These non-savvy users don't care about any distinction between original iPhones and 3G or 3GS iPhones, or amortization periods or anything like that.)
Rather than charging some iPhone owners for something that is free for other iPhone owners, it would probably less incite less resentment to simply say that the new OS is simply unavailable to original iPhone owners under any circumstances, but still free for newer owners. While there's still a double standard, it's a double standard which would probably be easier to accept.
Meanwhile, on the other side of the ecosystem, iPod touch users are already used to the fact that they have to pay for every new major version upgrade of the OS. So first generation iPod touch users probably would have been willing to pay for the upgrade to OS 4.0 as well. But that would also serve as another way to alienate original iPhone owners: first generation iPod touches are allowed to purchase the new OS, but first generation iPhones are not.
Again in an effort to avoid inciting too much resentment, the same standard will probably be applied to original iPod touch owners as to original iPhone users: original iPod touches will not be able to use the OS, but newer iPod touch users will be allowed to purchase it as always.
It would be perfectly reasonable on Apple's part to compromise between adhering to their established plan for implementing their interpretation of GAAP, whilst minimizing the number of people who end up getting pissed off in response. And it also presents a perfect opportunity to make a clean break from the older hardware and introduce kick-ass new features which wouldn't have performed very well on the older machines anyway.