eavesdropping

I've skimmed James B. Rule's "From mass society to perpetual contact" in Katz and Aakhus Perpetual contact. And there on page 253 he says this thing that I don't quite understand: "Everyone has noticed the irritation of restaurant diners on being subjected to conversations carried on via mobile phone by other diners". Rule compares the mobile phone (and mobile conversations) to an invasive and uninvited plant. Others are offended by people who have private phone conversations in public at all. In public spaces, why is it so much worse to have a conversation with an absent other than with someone who's present? What difference does it make for the eavesdroppers?