Why is there a difference between these two situations?

Consider how changes in information technology have caused a dilemma concerning ethical issues in organizations. Most people would agree that organizations have the right to monitor employees’ personal usage of company assets. This applies to logging Internet usage and the ability to access and read employees’ emails. However, it is not generally considered acceptable to open and read someone else’s letter—even if it was written on company paper. Why is there a difference between these two situations? What are the ethical implications here?