Why does Huck begin to feel bad for deceiving the sisters?

The night of the doctor's warning, Joanna and Huck eat together, since they are the youngest two people present. She asks him all about England, and Huck lies to her in order to sound knowledgeable. She catches him in several of the lies, and Huck keeps pretending to choke on a chicken bone in order to think of a way out. Mary Jane overhears Joanna telling Huck that she does not believe him and makes Joanna apologize to Huck for being so rude. Huck decides he cannot let the King and Duke steal the money from these extremely kind girls.