If Microsoft supported older versions of Windows with the newer versions of IE, and pushed 8, 9, 10 etc. via Windows Update, this wouldn't be as much of an issue. However, I think that what they are trying to do is force people to either (a) take the upgrade path to Windows 7 or 8, or (b) leave behind the latest software like Office >2010 and IE >8.
The problem is, upgrading Windows (any OS, really) is so fraught with complexity (and spending) that most people and IT organizations would rather stay on a nice, stable Windows (i.e. XP) and deal with the older software. (Especially nontechnical people who just have Win XP because that's what came on their computer.) Unfortunately, that means the developers have to pay for it.
The problem is, upgrading Windows (any OS, really) is so fraught with complexity (and spending) that most people and IT organizations would rather stay on a nice, stable Windows (i.e. XP) and deal with the older software. (Especially nontechnical people who just have Win XP because that's what came on their computer.) Unfortunately, that means the developers have to pay for it.