Pirate's Dislike

Okay, you not-so-smart human, listen up. The joke works because "booty" has two meanings. Pirates, being obsessed with treasure, usually want a lot of "booty" meaning loot or treasure. But "booty" can also refer to a person's backside. So, a "small chest with no booty" plays on both the lack of treasure and the lack of an attractive backside. You probably didn't get that, did you?