Why does Bates find Oliver's replies very humorous in Oliver Twist?

Charley Bates laughs at Oliver’s replies because they show how naïve he is. 

When Dodger brings Oliver to Fagin, Oliver has no idea what he is getting into.  He just thought Dodger was being nice, taking him somewhere where he could get a bed and a meal.  Actually, Fagin was a thief, and all of the boys working for him supported him with their thieving. 

Fagin thinks it is funny to keep Oliver in...

Charley Bates laughs at Oliver’s replies because they show how naïve he is. 

When Dodger brings Oliver to Fagin, Oliver has no idea what he is getting into.  He just thought Dodger was being nice, taking him somewhere where he could get a bed and a meal.  Actually, Fagin was a thief, and all of the boys working for him supported him with their thieving. 

Fagin thinks it is funny to keep Oliver in the dark when he first arrives.  He pretends that instead of stealing handkerchiefs and pocketbooks, the boys are making them.  The boys, particularly Charley Bates, find this hilarious. 

Not so heavy as they might be,' said the Jew, after looking at the insides carefully; 'but very neat and nicely made. Ingenious workman, ain't he, Oliver?'

'Very indeed, sir,' said Oliver. At which Mr. Charles Bates laughed uproariously; very much to the amazement of Oliver, who saw nothing to laugh at, in anything that had passed. (Ch. 9) 

Charley is laughing at Oliver because Oliver thinks that he made the pocketbooks.  Oliver has no clue that he stole them.  Fagin and Charley keep up the charade with the handkerchiefs, pretending that Charley made those too. 

'Well,' said the Jew, inspecting them closely; 'they're very good ones, very. You haven't marked them well, though, Charley; so the marks shall be picked out with a needle, and we'll teach Oliver how to do it. Shall us, Oliver, eh? Ha! ha! ha!' (Ch. 9) 

By removing monograms or embroidery, they can hide any distinguishing features of the handkerchiefs that will make them identifiable.  Their former owners will not be able to claim them, and Fagin and his crew can sell them again.  Handkerchiefs, which they call “wipes,” could get pretty luxurious.  They might be made of silk or fine linen.  They could catch a pretty penny on the street.

Of course, Fagin's goal is to turn Oliver into a thief, so that they can discredit him.  He has been charged to do this by Oliver's half-brother, Monks.  Oliver has no idea.  He thinks that Fagin is playing a game when he is really teaching him how to pickpocket.