If the table next door was having an obviously private discussion they'd probably be a bit put out if you started offering your opinion. People know their conversations often aren't secure, but then there are certain social expectations. Sometimes it's just expected that other people will politely ignore their conversation. If you told people that you had been spying on them I guarantee you that they would not blame their tech. They would place the blame squarely at your door for having listened in on something you shouldn't have.