Surfing the web on the job means that you're a risk for them knowing exactly what you do on the company's internet. It's like talking on the phone with the old-style landlines...someone could listen in from within the circuit. That's not at issue...
Using another example...You're at home watching TV (for the sake of this example let's say that your tv can be seen out of the window that overlooks a street or other public spot). You're watching porn. You're an adult and you paid for the video/channel, so you're well within your rights. You don't think it should be public consumption that you watch porn and when you watch it, so you close the blinds so no one can look in. Now, in trying to get a job, the company says that you have to keep your blinds open to them so that they can monitor what you watch on TV.
Even though yeah, you'd be an idiot for falling for their demands even in a tough job climate, the fact remains that it would still be illegal for the company to do so--because you're afforded the right to be able to close your blinds. Further, I think the EEOC states that in this case it would be illegal at worst unethical at best for a company to deny you the job because you refuse to allow them to peek in your windows.
The facebook cases are not so different than this. I think an example needs to be made, and soon.