This is not an issue about which OS is better. Thats a red herring.
This is about 2 separate issues
1) An additional cost of upgrading the OS that was not made clear earlier, nor factored in on users original estimates when pricing up the overall cost for D3.
2) A migration path/upgrade from 98/98SE/ME to XP/2K that although may appear simple to the seasoned techie may be more daunting to a casual user, who just wants to play the game on release. Many people who bought their PC's may have had 98/ME pre-installed and have never been through an upgrade process.
BTW I have XP Pro so I'm not getting dragged into an OS flame war.