@Soulfox -- True, but there are ways to get rid of files you don't want and cover your tracks. People can destroy hard drives so they can never be read again and there are ways to cover your tracks online through proxy servers.
Bear in mind that I am not advocating doing illegal things and taking steps to evade computer forensics experts. What I am saying is that some people have very good and legitimate reasons to keep their privacy intact when working on computers. There are ways to remain anonymous online and to keep private activities private.