With free or 30 dollar major OS upgrades, upgrades that are consistently better than their predecessors (or at the very least (Lion?) not noticeably worse!), great and constant innovation to make the upgrades compelling and appealing, smooth/streamlined and automated "SP" deployments over the net, and many other things that I won't bother listing, they ensure that their users are, by and large, using the latest version of their OS.
They can break compatibility every time, for all I care. But when Windows 7 (or for all we know, Windows 8 too) provides no real enhancements over XP (at the very least, in the eyes of a consumer) except bloat and a confused interface, then and only then do I complain about how "evil" they are for (possibly purposely, if you look into the details of the hack I posted in another comment, you'll see why) breaking support for older operating systems.