As many people would put it:
It's funny because it's true! Hahahahaha!
There's nothing inherently funny in the sentence. Maybe a bit of exaggeration (obviously not everybody dining there is a murderer), but that's it. This same sentence, said by anybody else, or said by Brennan himself but out of a comedy context, wouldn't make anybody laugh.
However, people laugh. Why? Because a comedian just said "something funny".
During their shows, stand-up comedians make use of a wide range of vocal inflexions, face gestures and other visual and auditive cues to let people know when they are supposed to laugh. It's part of a comedian's work; the audience knows it, and they know the cues and will readily cooperate when they have to. And more importantly, people laugh because they want to. They paid for a comedy show, they are in the mood of having fun and laughing, and they'll laugh at every single chance there is to (usually following the aformentioned cues).
But seriously: this doesn't really have anything to do with English Language Learning (despite your comments). It may have something to do with American Culture Learning, but this is not the appropiate site for questions about that.