Why would one need to trace somebody's web activity two years into the past to prosecute child pornography?
If the government needs a witness to lie for a criminal prosecution, a fishing expedition of internet activity going back two years might provide leverage.
Suppose a prosecutor is under pressure to pin a murder on a suspect. An old trick would be to coach a little old lady to identify a former associate of the suspect, then coerce him to testify. If the prosecutor could find out that the associate was married to an illegal alien, he could have the wife arrested and the child taken away to force the innocent man to lie in court. (I saw that on TV.)
Information on an individual's internet activity could be useful for a corporate or government official who wants power over that individual. If the information were kept, I'll bet it could be accessed without a warrant.
[TELECOM Digest Editor's Note: There is a name for this kind of activity: 'Prosecutorial Misconduct'. The Chicago Tribune did a five part series on this subject in 1999 which led to the disbarment of five or six attornies (who were prosecutors) in Chicago, two dozen judges of the Cook County Circuit Court and in 2002, a one hundred percent cleanout of Illinois 'Death Row' by the governor at the time who frankly stated he had considerable doubt about the guilt of many of the Death Row inmates. Needless to say, police were furious at having this happen, and the prosecutors screamed rather loudly also. PAT]