The feelings that an eavesdropped person often feels are that of trespass, betrayal, breach of trust and confidentiality, hurt and humiliation. These feelings are difficult to dispel even with an apology, let alone the eavesdropped person being able or willing to shrug or laugh them off. On a moral scale, eavesdropping is about on the same level as following a person everywhere they go and looking at them, laying your hand on a sensitive part of a person’s body, or having an affair.
Also, using lack of knowledge to justify eavesdropping would, by extension, justify other surreptitious wrongs, such as cheating in a relationship, being a peeping tom, shoplifting without getting caught, cheating on a test or “buying” a paper for school, or a crematorium keeping a body for research and giving the family pretend ashes. No knowledge does not mean no victim. Besides, if you had any respect for the people having the private conversation, you would WANT their conversation to remain private between the two of them.
