To side with the retailers I have to ask what exactly you expect.
The average supermarket will have 10-20 thousand product lines in stock.
They also don't have national pricing, some products will have different prices in different stores, and prices will change at least twice a week. So your looking at around 1/2 a million pricing records.
That's a lot of prices to organise, and get right.
Now most retailers are not out to deliberately defraud customers, but mistakes do happen, and I would think that when there's a queue of customers a mile long, fixing that pricing mistake you pointed out just gets pushed further and further down the to-do list...