I can believe customers act that badly, but sadly I have more trouble believing the 'good' stories with happy endings. I think many of those are embellished, or include a lot of wishful thinking and things the writer wishes they have thought of at the time.
I've worked food service, call center and customer service for a publishing company. In my experience, 25% of people are nicer than nice, 50% are pleasant, 14% are neutral, 10% are not always reasonable, and 1% are just plain awful.
I really did have one woman get in my face and cuss me out because we were out of mint chocolate chip ice cream. I had offered to make her a shake with mint and chocolate chips in it for the same price as a cone, but she wanted a scooped cone and nothing else. I was an incompetent certain-part-of-the-anatomy because we had just had a soccer team come where every kid wanted a mint chocolate chip cone.
But the sad thing is, the rudest population of people I ever dealt with was when I worked for a crazy, delusional woman who had inherited the company from her mother. All her friends/colleagues were the same silver-spoon-born, work at mommy/daddy's company sort of executives. (So firmly in the camp of "They should know how to behave") I had to fill in as a receptionist once and those people were just plain AWFUL. I'm not going to generalize to "rich folks" but that particular group was just snotty!