How do Huck's interactions with the Duke and the King help him grow morally?

Huck grows in lots of different ways from his interactions with the Duke and the King. For one thing, he becomes much less trusting, realizing just how easy it is for someone with a smattering of book learning to con ignorant folk out of their hard-earned cash. But it takes him a while to understand this. Initially, it's kind of fun to hang out with these two notorious charlatans. They're colorful characters with the gift of the gab and break up the monotony of Huck and Jim's long raft journey down the Mississippi.
In some respects, Huck has similarities with the two con men. Like them, he's a social outcast, someone who doesn't really belong in civilized society. The difference, however—and it's a big difference—is that Huck is fundamentally decent and honest, whereas the Duke and the King are just a couple of crooks. Through his interactions with them, Huck comes to realize that, although he can't actually live in the civilized world, it's still very important to have rules and regulations. Leading an easy life with plenty of grub may have its attractions, but it still has to be an honest life. It's one thing to live outside the law, as Huck and Jim do, but it's something else entirely to live by breaking it, as is the case with the Duke and the King. Huck learns this lesson well, as he sees the two old hucksters getting their just deserts, tarred and feathered by a braying, angry mob.