This is being sold using fear:

...company spokespeople emphasize liability. What happens if one of your employees freaks out, comes to work and starts threatening coworkers with a samurai sword? You'll be held responsible because all of the signs of such behavior were clear for all to see on public Facebook pages. That's why you should scan every prospective hire and run continued scans on every existing employee.

In other words, they make the case that now that people use social networks, companies will be expected (by shareholders, etc.) to monitor those services and protect the company from lawsuits, damage to reputation, and other harm.

http://www.schneier.com/blog/archives/2010/10/monitoring_empl.html