alt Hacker NewsThere's no "just". It takes resources to be scanning your receipt for discrepancies and/or running your own tally. And there are a few examples in the article referencing stores refusing to adjust prices, or of people noticing on their receipt that they were defrauded and the store refusing to reimburse them.
Resources to read the receipt? Are you saying poor people can't do math? Honestly, how much effort does it take to look at your receipt and look for errors? If you are really on a tight budget, I guarantee you will be looking over your receipt.
I have watched countless people shop with a calculator or pen/pad to make sure they stay on budget. It is not hard.