To get a little pedantic, assuming such a promise existed, it actually doesn't mean as much as most people think.
A merchant saying "we won't sue you in civil court for the missing money" does not prevent the local government from criminally prosecuting that same person for theft.
American TV dramas often show the police asking people "Do you want to press charges?", but the idea that the question matters is a myth, since victims of crime don't get to decide that. At best, it's a terribly misleading shortening of: "Just for my own private curiosity, do you plan to lobby or press upon your local government officials into pressing charges?"
Exactly. Retailers generally assume liability for honest mistakes made by their system/employees and sometimes even their customers. However, when people knowingly exploit a loophole for financial gain it becomes fraud. Here's a real example - https://www.wfmynews2.com/article/news/local/greensboro-woma...
IANAL but I'm confident the legal answer is "yes", the same as if someone was using sleight-of-hand with objects at a cashier-and-conveyor-belt checkout station.
Whether charges are brought and how easily the case can be proved is another matter, but the intent is what makes it a crime.