I like this analogy. We all know what goes on inside a bathroom, it's not really a secret. But it is private. There is a difference between secrecy and privacy, and this analogy captures the difference well.

I think I first heard the analogy in Cory Doctorow's presentation The Coming Civil War over General-purpose Computing, which was ironically given at Google. I highly recommend people watch it.

Or "why lock your car doors when stealing and hotwiring is a crime"? Sure, people have convertibles and jeeps with no doors, but they also won't leave anything valuable out in the open.