Well, they can do it, but it might mean leaving features out. Although it's pretty shocking that they don't already know this! But then this is all Apple do. The older phones get the new OS, but with several bits cut out, either because the hardware isn't there, or because Apple want to keep it for the new shiny-shiny. Perhaps they were being very cheapskate in not giving 1GB of RAM on all the models, it wasn't going to add that much to the cost?
But then Apple did exactly the same with the original iPad, where they gave the phones 1GB or RAM and the iPad 1 (launched 2 months before) only 512MB. Which meant that the iPad couldn't take the new version of iOS (iOS 4 if I remember right) for about 4 months after the phones got it, and then ran a bit slowly after that, and particularly the "upgrade" to iOS 5.