Hemingway Hate Club discussion

Hi
Could someone explain to me what the joke is in this dialogue (3rd chapter).

"Oh, priceless," I said.
Brett laughed. "It's wrong of you, Jake. It's an insult to all of us. Look at Frances there, and Jo."
This for Cohn's benefit.
"Its in restraint of trade," Brett said. She laughed again.


Why is it an insult to anybody? What's the issue with Frances and Jo. Why is it for Cohn's benefit? And how is it in restraint of trade?

Thanks.

Who knows Hemingway could never make a good joke anyways