When it all comes down to it, a Business exists to make money. Maintaining a margin of profit is more vital to a company or corporation's existence than any other factor. Sure, the legality of the companies' actions and the well-being of its employees are very important as well, but in a free market economy such as the United States', if a company's not making money, it's not going to survive. It's as simple as that.
That's why monitoring in the workplace is so important. It's one advantage in a never-ending struggle to stay on top. If I hire someone to do a certain job, you better believe I'm going to do what I can to ensure that he or she does it. I don't want my employees using company property on company time for personal matters. That's not what they're here for. And if they are doing so, I want to know about it.
Of course, my employees could be doing far worse things than using my phones to dial long-distance friends. "The most common reasons given by employers for electronic monitoring," said one survey, included "investigation of industrial espionage" (2). If I don't protect my company secrets, they won't be secrets for long. It may not be in my best interest to treat "all employees like potential criminals", but many business owners feel that "they have no other choice in order to keep up with their competitors" (10).
Also, if my employees are making honest-to-God mistakes which they are unaware of, I want to be able to catch them before they hurt productivity too much. If employee performance can be monitored, I can find out where it needs to be improved.