Why can X.11 on Mac OS/X only run on Mac 10.3???

They finally have X-Windows on Mac OS/X – it is free but when you download it it demands to run on Mac OS 10.3 (which everyone ha to buy). Great marketing on the part of Apple :)
But there is a version at http://sourceforge.net/projects/xonx/ that was an earlier release that I downloaded at the Amsterdam airport – ha ha!