A private conversation is one that is intended to occur only between the parties involved. Your being able to hear it or not is irrelevant if the intention and expectation was that the conversation is meant to be between specific parties.
I understand this is not completely black and white, but fail to understand why anyone should take offence at a conversation they were not meant to be a part of and where they don't understand the history and social dynamics of the people involved in the conversation.
I can think of all kinds of situations where a private conversation, if overheard and taken out of context could be devastating to those involved.