Not everyone is a highly technical person and to lump everyone into this category is not right. Try explaining this to elderly users.
For example, you might post pictures of your college reunion, and not share them with work colleagues because you want to maintain a more professional kind of relationship with them. But you shouldn't expect any confidentiality for things that happened at a 200-person event.
A real world example is meeting up with your friends at the bar and running into a coworker. You aren't prohibiting your coworkers from seeing you out with your friends, but you also aren't affirmatively sharing that part of your life with them. And what is appropriate to coincidentally be seen doing is different than what is appropriate to actively share in certain contexts.
It may feel that way, but it is an unrealistic expectation. Something shared with hundreds of people is effectively public information. To believe otherwise is a fantasy.
In “the real world”, we intuitively understand this: a secret told to two dozen people is a secret no more. But the illusion of control on social media sites seems to convince people otherwise online.