I think part of the problem is, is that people are often friends on facebook with people who are NOT their friends. In order for your employer to take disciplinary action for something you wrote on facebook, someone had to alert them. If your page is completely private, it was one of your so called friends on your friends list. If your page is public, STILL, someone had to alert them. They didn't just stumble onto facebook, enter your name in the search, then go look into your page. If your page is completely private and you aren't friends with anyone from your job, nobody can see what you write.
Either way, better to be safe than sorry because you never know who has an ax to grind or who wants to see you fail. Never underestimate how far someone is willing to go to stab you in the back. Better to not post anything about work that could be used against you.