Because this would allow these millions of people to continue using this fork of XP rather than eventually buying a MS operating system.
An open source XP would get many hackers interested in Windows, something they desperately need.
They are definitely making money on closed-source operating systems and operating system tools:
http://www.microsoft.com/investor/EarningsAndFinancials/Earn...
It wasn't free--it took a couple of hours of my time!