Google is doing this to build a migration path for users XP to Chrome OS.
By 2015 Google will have a mature Chrome OS, and a huge number of XP users that have no migration path. This is a huge opportunity for Google to win these people over as users -- first as Chrome users, then as Chrome Apps start becoming plentiful through Chrome, XP users will have a way to use modern services while skipping over WIndows XP and the non-existent modern services that will be available for it.
The next step for Google is to get Windows to boot directly into Chrome instead of WIndows XP loading the Explorer Desktop. This is a trivial configuration. Every Windows XP box can be converted into a Chrome OS box once Google gets all the right pieces in place. This is a path to wean users off of Windows and onto Chrome OS.
Of course, when users have to purchase another machine, they will already be familiar with Chrome OS.