In this comic situation, I'm more inclined to be (also) angry at the boss firing them instead of being (even more) angry at the shoplifter.

Most of the loss is from alcohol, meat, seafood, frozen foods and cosmetics. These are high priced items. Losing 50 dollars worth of shitty art supplies is a lot easier to handle than losing 500 dollars worth of steak and crab legs. That happening several times a day every day of the week for years puts a damper on the amount of money that can be used for maintenance, repairs and forces the prices of the goods up dramatically. And that could kill a store's ability to remain open should corporate deem it too costly to run without any real return. In the end the only people who hurt are the local employees and the people who use that store.