The point is having a Pentium III Win 7 machine covers using a Pentium 4, Athlon XP, and all those other old computers to install IE9 - covering the biggest fraction of users through backward compatability.
You can run Windows 7 on pretty much anything that will run Windows XP. "It costs money to upgrade" may prevent you from doing so, but at this point, after a decade, it's not Microsoft's problem if you're not willing to spend the money. They're not obligated, nor should they be obligated, to avoid using their new stuff (Direct2D, for example) in order to continue to target XP users.
