To help combat piracy I would imagine, so if you buy your computer from someone, and you start it up and it says evaluation copy you know something is amiss.
I have a Toshiba Satellite U505 that I would like to try out windows 7 (or even windows 8 if it can run it) on. Currently it is runnning Lubuntu, so nuke and pave is my best option. It came with Vista ...