When you work in the service industry long enough, you can start to believe that customers intentionally scream and holler at you for their own sick pleasure. If you work at a busy restaurant or a retail store during the holiday season, you're bound to get treated like you're sub-human at least once per shift, and most of the time, it's not because of anything you did. It's because the kitchen added vegetables to their Kung Pao Chicken when they specifically asked the waiter for "no veggies." It's because they feel emasculated, and the only way that they'll feel like a big man today is if they scream at someone with even less power than they do.
While it might be a Pollyanna perspective, I don't think anyone intentionally goes out of their way to make the lives of service workers miserable. It's much more common for a customer to be pushed to their limit in the moment, simultaneously making both their life and the lives of the employees miserable. In those situations, the only way out is through, and though raising your voice with service workers isn't always the best practice, there are times when it's the only way to solve both the customer's problem and the employee's problems.
While it might be a Pollyanna perspective, I don't think anyone intentionally goes out of their way to make the lives of service workers miserable. It's much more common for a customer to be pushed to their limit in the moment, simultaneously making both their life and the lives of the employees miserable. In those situations, the only way out is through, and though raising your voice with service workers isn't always the best practice, there are times when it's the only way to solve both the customer's problem and the employee's problems.