Also borne out later in the novel when it turns out that the detective in charge of the investigation bugged everyone’s phone.
As to the OP, I’m pretty sure it’s a combination of “Lots of people and background noise”, with an element of “And most people like food and/or alcohol”.
Let’s face it, if you’re going to discuss your Secret Plans™, then it’s a lot more comfortable to do it at the local pub or a restaurant/cafe than in a carpark or somewhere like that.
A new book I was just reading on the Battle of Midway and events leading up to it, mentions that a couple of top military strategists met with Jimmy Doolittle at a restaurant in 1942 to discuss plans for the Doolittle raid on Japan. They apparently kept their voices low.
People will overhear and report allegedly dubious conversations at restaurants. Shortly after 9/11 there was an incident where (as I recall) a couple of young Arab (or Arab-looking) guys were arrested because someone at a Shoney’s (!) supposedly overheard them plotting something dire. And awhile back, Mrs. J. and I were eating at a Mexican restaurant in Texas and discussing our Sally’s behavioral problems and why it was necessary to leave her at home in a large cardboard box while we ate out. At some point during this discussion I became aware that a woman in the adjoining booth was giving us a death glare. I suspect she did not realize that Sally was a cocker spaniel. 
I find it worse when people discuss confidential stuff in emails and text messages. Overheard conversations are just hearsay, but texts and emails are evidence and don’t go away when you delete them from your phone or computer.
We just had a recent murder where the prosecutor introduced transcripts of the texts of the defendants discussing how they were going to commit the murder.
It’s one thing when they make a not toward discretion. They talk in low voices, they’re in a crowded club and they lean over near to talk in each other’s ear.
It’s quite different when they just blare it out without any regard to who is around. Like the episode of Covert Affairs, where Annie is talking on the phone to her sister, and how she can’t tell her sister where her house is because it’s a CIA safehouse. She’s on a mission, trying to keep a low profile, and certainly needs her cover intact, yet just blares that out because her contact isn’t there yet.
Or two people in a bar on Terra Nova are discussing stealing stuff to give to the Sixers. Out loud, with no apparent regard for who might be listening.
That grates.