There are some pretty reasonable objections to Facebook revealing things unexpectedly: this isn't one of them. If you wish to control who reads a message you've left on a someone else's quasi-public wall then Facebook certainly isn't the service for you.
While I understand what you're trying to say and agree with that, I think that the issue people have is that Facebook is giving them a mechanism to exert that control and then that mechanism doesn't work.
I don't use Facebook. But other people in my home use Facebook which means they're tracking what I do—as a non-user and someone that hasn't agreed to their terms or privacy policy—anyway. The only solution to this is for Facebook to stop tracking people when they log out. The government needs to step in because they obviously won't do that on their own.