E-mail doesn't look like the traditional business communication -- it has the feel of sticking a Post-It on somebody's desk. E-mail can be sent without a lot of reflection. Most employees don't really think of the fact that when you send an e-mail there will be a copy on your computer, the network backup tape, etc. It's frequently harder to get rid of an e-mail than a written document.
Employers must have the ability to review everything on their computer systems to make sure that there's no illegal activity being conducted. If someone conducts criminal activity using an e-mail system, unknown to the company, the company's e-mail system can be subject to seizure. Or an employer may be sued in a breach of contract case. As part of that they're going to have to go through a lot of employee e-mail.