Fair enough. I'll rephrase. Why does it make the usually unacceptable acceptable?

Perhaps too often I'm on the wrong place at the wrong time but I hear plenty of half conversations in public (e.g., on the train) that would not happen if both parties were in the same row.

Too much volume. Questionable subject matter. Far from urgent. The rest of us don't need to hear that. Do they?

That part I'm on board with. Work conversations, kid conversations, spouse conversations that would never be aired in public otherwise, and now I'm privy to toilet-stopped-up issues on the bus.