Tom's being very cunning here, as always. He knows that the only way he's ever going to get Huck to go back to the Widow Douglas is by making an appeal to his sense of adventure. Running away from home is all very well and good if you want to be a pirate, but it's no good for being a robber. Tom has clearly had his vivid imagination fired up by tales of so-called gentleman thieves. These men are a cut above the kind of cutthroats and scurvy dogs who sail aboard pirate ships, and Tom wants to be a part of their exclusive company.
Boys of that age tend to feel the need to be part of a gang—and the more exclusive the gang, the better. If Huck wants to join up with Tom's gang of robbers, he's going to have to pursue a respectable, civilized lifestyle, at least during the daytime.