It is pretty obvious that most who write and enforce the laws when it comes to illegal pornographic material are out of date when it comes to how modern technology works.
In this day and age anyone can stumble across illegal pornographic material (including child pornography) without even meaning to. It can be attached to a virus, or be part of a file you download that seems completely innocuous like a home video sent by a friend or a picture off an imageboard. A "trusted" friend could easily download without one knowing.
Just because a person owns the computer the material is on, should not mean the person is guilty of owning the material. Law enforcement should see timestamps of files as well as length of views to determine if the computer owner is in fact guilty of possession or not.