Why (pretend to) be moral? Joyce thinks that "Hume's answer to a sensible knave" is "roughly correct" (p. 210). That is, bad people don't get to enjoy having a good conscience or the satisfaction of having been good, and they run the risk of being caught, which isn't likely to go down well. (See Survivor for numerous examples of people losing because of perceived untrustworthiness or plain nastiness.) So most people are better off behaving morally.
And they are more likely to do so if they not only know that moral behavior is in their self-interest but tend to think that it is necessary or somehow obligatory. So we should get in the habit of "employing the fiction of morality" (p. 219). Indeed, we should raise our children to believe in morality, and only break it to them later, perhaps when they are old enough to do philosophy, that it is a myth.