They certainly don't own it. This is interesting considering that the CSL office was actually opening letters that were not stamped to see if they were personal. As far as I know, they have stopped opening unstamped letters. It is similar in that in that it is a right to privacy issue. Other companies scan their e-mail for keywords such as product names and such.
Many sites also scan for worms attached to e-mail.
The email legal cases that I have heard are companies that read personal email for fun or managers who want to get the dirt on their employees. This is a definate privacy issue. Sending viruses, passwd files, and proprietary information is not as clear cut. It is similar to sending explosives or even volatile material through the US mail. I am not trying to defend the university.