Re: Wait, hear that?
Windows 7, Windows 8 and Windows XP share such a large code base that I'd imagine that if you have a bug that applies to a feature in Windows 7, and that same feature exists in XP, there is a strong chance the same bug exists as well in XP. In a way, you can see MS' position, but you can easily see the counter point as well - these are bugs which you have then fixed, but can no longer be arsed to merge the fix back.
In an open source project, removing support from a release with this number of active users would be shouted about on the mailing lists, with two inevitable conclusions - firstly, support would be re-enabled, probably with some new members of the security team who are interested in that release and will do release management for it, and secondly, that the project would have some navel gazing as to why so many people are still using the version from 2 releases ago.
Apple get around this issue by having a strong commercial relationship with their customers, and by regularly updating the OS for nominal fees. An OS X upgrade is less likely to leave your OS half working, as I have seen numerable times from XP->Vista and XP->7 migrations (so much so that now, if I'm asked if I can "quickly help out" and upgrade from XP to anything, I'll refuse and insist on a re-install),
Apple can do this because they don't make money from the OS, they make it from the hardware and ecosystem - someone buying a Mac is in the market for an iPhone, iPad, Apple TV, special mice, ridiculously expensive monitors and so on. MS only make their money from people upgrading or buying new machines.