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On the one hand, I work retail and know that this is literally "never" the responsibility of the grunts. It's too dangerous and generates a bunch of bad optics for the store over a trivial amount of lost product.
On the other, I've seen what fast food workers have to go through. Every. Fucking. Day. You get "an" order wrong out of 100, and that person is going to act like you set out with the intention of sabotaging their metabolic massacre. You will be cursed at, yelled at, insulted, and intimidated, often by someone who's too fat and gross to really make judgements about other people.
Couple that with some haughty little suburbanite tramp taking free soda (like it's not literally a fucking dollar and she isn't walking around in an outfit that costs more than your car), and yeah, I might be ready to fucking snap too.
Please keep in mind that food staff members are still people, and that people are perfectly capable of going postal when they take too much shit.