If you knew nothing about the book, other than the title, how would you interpret it? "Prison" sounds closed, dark, regimented, disciplined, monotonous, restrictive and isn't a place you'd like to be.
Then, as you read the book, you find out that the setting of the book is actually a candy store. Candy stores could be described as fun, bright, spontaneous, a treat, for children, happy places. The title and the setting are therefore in direct contrast.
Would you have guessed that the book was about a family working in a candy store from the title?
For Tony, the store is a prison. He's stuck doing twelve-hour shifts, with little personal time (an hour in the afternoon, in which he naps). There's no possibility of 'escape'. There are other hints of 'prison life': Rosa gives Tony a new name (Tommy), just like prisoners being referred to by an ID number.
There are references to petty, and more serious, crimes that occur during the novel.