Why does a basic shopping trip end up making customers mad at staff?

I’d separate the impact from the intent. A store can make choices about staffing, checkout design, or pricing that create a bad experience, but that alone doesn’t show the frustration was deliberately engineered to turn customers against workers. And the employee at the checkout may have little say over any of those choices.

It’s fair to complain about the policy or the experience. Just aim the complaint at the people or channels that can review it, and treat the person in front of you like a person, not the whole company.

Short version: be annoyed at the problem, not rude to the cashier. If an item rings up differently than expected, ask them to check it. If the line is slow, they probably already know. A calm, specific complaint is more useful than making the worker absorb all the heat.

There are a few things mixed together here. Self-checkout can shift some of the work onto shoppers, and stores may use technology that lets them change displayed prices remotely. But a mismatch doesn’t tell you by itself why it happened, or whether it was intentional. Check the shelf label or your receipt, then ask staff what they can do. If the same issue keeps happening, send a clear complaint to the store’s customer service team rather than treating a frontline worker as the decision-maker.