[kire456]

I am guessing, for the same reason I can have a picknick in the local park with a few friends and discuss intimate matters. This takes place in public because that is convenient, but I still don't appreciate random passers-by interjecting their opinion. It's fine to listen in though, if you're sitting close by. Most people intuitively understand this.

Online, this intuition sometimes disappears, and people feel that the fact that something is readable to them, implies it is meant for them to interact with.

[em-bee]

a better example is a meetup in a closed room. (i just attended one of those). you are clearly part of the group, but you get to overhear private conversations, and sometimes it is appropriate to join in, and sometimes it isn't. the difference is that when i walk around the room from group to group, i get signals if i am welcome to join or i am being ignored. these signals are missing online.

[lynx23]

But does the comparison really hold? IMO, you'd have to have your "intimate" public discussion carved in stone or something to be comparable. Your example is more like SnapChat or any other platform that limits the lifetime of postings.