We cannot rely on stores to make sure we only purchase kosher l’Pesach foods.
Let’s say a store has dedicated Pesach aisles and refrigerator/freezer cases and they are careful when they stock the shelves and they verify that the Pesach order really contains only Pesach food and everything is great before the first customer walks in – who is responsible for making sure a “helpful” shopper doesn’t put something back in the wrong section?