Apple and Microsoft are quite capable of creating their own virtualization software and/or tying up VM software makers in costly court proceedings. So they exist as long as they play ball.
On PowerPC processors, there was VirtualPC, which allowed Windows to run on Mac's although it was slow it worked. Microsoft bought them and killed it. Now VirtualPC is used to allow Windows 7 Pro to run XP.
Apple is new to Intel processors, and now it's customers are seeing the benefits of virtualization software, so they let Snow Server, but not Snow client because they likely thought there wasn't a need.
Microsoft see's a advantage to expand it's influence over Mac users so it allows Windows to be virtualized and even Bootcamped on Mac's.
Apple see's a benefit to allow Lion client to be virtualized in Lion client only.
It takes time for Apple to come to it's senses and allow certain things here and there, it's looking for what benefits Apple and not always necessarily what benefits it's customers.
Apple is likely terrified OS X client will be weaponized as a vm on Windows, surely would kill hardware sales fast.
Also Apple considers most of it's users newbie computer users, so always strives to create a ToyStory Disney World like computing experience, it's "Pro" users are almost second class citizens compared to the majority of those who all they know how to do is poke their fingers at a iOS screen.
I've been a long advocate of allowing OS X to be virtualized (but in a very hobbled fashion) on Windows PC's to gain swtichers and educate Windows users how OS X works.
Perhaps one day that will occur, and the previous EULAS again won't apply.