Who said anything about illegality? There are many perfectly good commercial and personal reasons for not wanting your computer to save away every file you ever access and every web site you ever visit. Heck, Mozilla just put a new feature into Firefox to _prevent_ it from logging what you do because many users wanted that.Obviously, if you are concerned that someone might be monitoring your workstation, it would be best to minimize traces of your illegal activities - hence no Zeitgeist for you. :P
If this goes into Gnome and can't be turned off, I'm not even sure we'll be able to justify using it at work anymore. For example, a while back I was given sample files from a customer which they required me to delete all copies of once we were done testing; how do I do that if the _windowing system_ is saving away copies in random places?