Epigraphs are like little appetizers to the great main dish of a story. They illuminate important aspects of the story, and they get us headed in the right direction.
If the people who came to town had wanted Bible readings, we'd have given 'em that. But nobody ever asked for Bible readings. They wanted booze, broads, and gambling, so that's what we gave 'em. - Murray Fredericks
What's up with the epigraph?
Well, it's true, isn't it?
After all, it's not like the dudes who ran Atlantic City were awful people or anything. Though they did some pretty shady stuff, they did it for one reason—to support their city. They only got into illegal activity in the first place because that's what the customer wanted, and the customer is always right.
This becomes especially true when you remember that most of the city's visitors hailed from Philadelphia. At the time, Philly was "a conservative and traditional town" (4.13) founded on strict Quaker morals. During the Industrial Revolution, however, new immigrants flooded into the city to work in factories. These blue-collar workers weren't too interested in going to church, though—they just wanted to have a "hell-raising good time" (4.14). Understandable when you consider how hard they worked.
So can you blame Atlantic City for giving them what they want? That would be like getting mad at Taylor Swift for incessantly instagramming photos of her cats—you can't blame someone for being who they are.