Because that's how it stops you making copies!
Dipping into the driver level (and in some cases below in the OS into direct memory management) is the only way that any copy protection technique can remain secure for over a day. Due to the extreme skills that some crackers display, such techniques as polymorphic and masked code have to be used to slow the process. Any protection is breakable with enough sweat, so the current aim of protection mechanisms is to allow the software to pass into obselence before it is cracked.
I'm far from saying that starforce is perfect, or that it is right for every situation - but I wouldn't let it stop me from buying something I wanted a bash on!
Interestingly, CDilla's solutions are becoming as invasive as starforce - they just keep quieter. :smilewink: