I say this as someone who worked food service and retail for many, many years and is fully aware of how awful and soul destroying working customer service can be, but:
I think people also often forget that power dynamics don't always fully favor the customer, and sometimes power dynamics can be tilted in the favor of the customer service worker and subsequently abused. Able bodied customer service workers can be, and often are, ableist to disabled customers. White customer service workers can be, and often are, racist towards customers of color. Customer service workers who are men can be, and often are, sexist towards customers who are women. Customer service workers who are non-immigrants can be, and often are, xenophobic towards immigrant customers. Shall I go on? That's obviously not an exhaustive list and I could very well go on.
When I worked retail I had a co-worker scream at a little old lady and kick her out of the store because she spoke to him in a language he didn't speak. I saw her face as she was leaving and she looked so shaken up like she was about to cry. I've overheard so many of my co-workers calling customers racial slurs behind their back. Once again, I could go on, but I feel like my addition here is already getting long enough.
Yes, you should be able to stand up to customers who are being unreasonable jerks, you should be able to refuse service to customers who are being unreasonable jerks. But the ones who aren't doing anything harmful you should still be nice to, not because it's your job, but because they're a human being.
