Do you memorize all the time whether you grabbed two or three bags of chips or exactly how many cans of beer etc?
How do you prove you didn't buy something? Or will they just accept your word? If anyone can just say whatever, then people will just ask for refunds of stuff. Will they check the footage in each case? But maybe it can work in the US. It sure as hell won't work in many other countries, where people look for loopholes all the time.