Because Gamespot is in bed with Microsoft. Nothing more, nothing less. And the only bit that's OS dependant is the crappy ass installer.
Anyone running UT2k4 on non-XP Windows, or Linux, or OS X can download from worthplaying and unzip it to get the exact same content (the WP file appears to be a zip, not an exe, so even though they say "all Windows" it's really "all platforms").
The point, however, remains -- it is a slimy thing to do, and there's going to be some people who will think they can't get it at all because they don't run XP.
No - most savvy gamers prefer not to install a bloated piece of turd on their computers. I am sticking with Win2K - at least I don't have to deal with product activation or the endless adds to sign on to Passport and Hotmail every time I turn on my computer.
When it says on the page that these maps are "Epic designed", does it mean what it says?