You can purchase XP seperately, so Apple's defense is, Microsoft is making more money, because people are *supposed* to buy XP if they want it on their Mac. Of course, we all know most of them probably bootleg it.
Edit/Correction: But they aren't selling the Intel-compiled version of OSX seperately from Intel-based Macs. Even if you had an Intel based Mac that comes with the OSX cd, it will probably still require reverse engineering, which is illegal.
Apple had the advantage of being able to compile OSX FOR Intel platform and therefore, creating a "security system" to prevent it from running on PCs without hacking. Microsoft didn't have that advantage, as they never foresaw Apple using Intel chipsets back in 2002(?) when XP was released.