You have two identical products, with the same price, next to each other on the same shelf. The box is damaged for one of them - it's the natural choice to choose the one that's not damaged in any way.
Now, if the damaged one is a little bit cheaper, you'll probably have a greater chance of accepting the product with cosmetic damage over the more expensive, "perfect" product.
I was just thinking -- if there was a way to somehow be able to offer instantaneous discounts on "damaged" products, supermarkets could probably avoid throwing away a lot of "damaged" goods.
But then I realized people would just start damaging them themselves, in order to get the discount.
Now, if the damaged one is a little bit cheaper, you'll probably have a greater chance of accepting the product with cosmetic damage over the more expensive, "perfect" product.