Sadly, stuff like power plants *are* on the public Internet
"Targets like power plants should not be on public networks."
I agree. Unfortunately, this isn't true. They've discovered, for example, computers in power plant control rooms on the public Internet. The recent Stuxnet virus that damaged the Iranian nuclear program is a great example of how difficult it is to secure these things.
Another thing we've learned recently is from Wikileaks: it takes is one insider with motivation to bring the whole thing down. In Wikileaks, it was one low level soldier who delivered thousands of confidential documents. I used to work at a managed network services company, and for at least one of the clients (a major, regional bank with thousands of ATM machines), I learned enough on the first week that, had I cared to remember the phone number for the backdoor line into a single ATM machine, and cared to remember the password, I could to this day take day the entire bank. If you ever get cynical about the goodness in the average person, you're wrong... our entire infrastructure from top to bottom, private and public and government, is secured by trust in people who we have no reason to trust.
The book "The Cuckoo's Egg" contains an excellent example of how easy it is to obtain the services of a good hacker and convince them to turn their attention to military and government targets for the purposes of a foreign enemy. In the book, the hacker was motivated by an expensive drug habit. This is why Alan Turing was pushed out of government service; while his sexuality was an issue in and of itself, more importantly it made him vulnerable to blackmail and extortion.
Indeed, if you want true security, you need to treat your employees VERY harshly, and get extremely involved in their personal lives. Do you have an employee who is cheating on their spouse or otherwise doing something illegal or immoral? Do you have an employee with an expensive drug or gambling habit (or some other illegal/expensive vice)? Perhaps you have someone in a sensitive position who has big bills or four kids in college all at the same time? Each of these employees is vulnerable to pressure from an enemy, and they may have been trustworthy when you hired them, but now they are a potential security risk.
It's unfortunate. I'm not advocating digging into the personal lives of people, incidentally. But that's another great reason why I favor a kill switch concept. If it's a choice between performing a full background and lifestyle check similar to a top secret clearance every six months on everyone (talk about "Big Brother"!) or being able to mitigate the damage of a compromised person... the kill switch looks like a much better option to me.
J.Ja
Contributr 
