I think the better rhetorical question is: why are some people so amazingly stupid, that they are incapable of telling the difference between these two scenarios? What is causing this stupidity? Is there anything we can do about it, and if there is, should we do it?
When Facebook purports to give you control over your data, privacy settings, then yes it is absolutely reasonable to expect that stuff to be private to the people you choose. Yes, we know that their impenetrable contracts give them loads of rights, but the site does not communicate that in how it works. Despite your colourful analogy, very little about how Facebook works would make a normal person believe that they are in a fully public place.
I know it's superfun to call people stupid, and it makes you feel really great about your level of intelligence, but I would put real money on you believing stuff about other fields (law for example) that you're not personally involved in that are simply wrong. People do not have time to understand everything and they have to live their lives using reasonable expectation. The reasonable expectation of Facebook posts is that they are private to your group of friends.