Who were you replying to? Well, to get to it, there are good customers, but then there are also those who go out of their way to be total assholes, and completely abuse the "always right" part.
I don't know how many of the latter exist, I haven't witnessed many in my neck of the woods, but I can't speak for all parts of the U.S. or other countries. Shaun has talked about his worst customers and I have to agree it does come a time when the manager needs to take control of the situation, so the entire store is not disrupted, and other customers not disturbed.
But...if the place is obviously full of people who don't give a rat's ass, then the customer does have a right to let the staff know that he is displeased. If it's clear neither the staff nor the manager gives a shit, then I definitely wouldn't condemn any customer for losing his temper and raising his voice. We're all only human. Still it's best to keep your head and express your displeasure in a cordial but firm manner.
What gets my dander up is when the staff and/or manager exhibits indifference, both in his response to my complaint, the tone of his voice, and his facial expressions. And that means giving explanations that sound more like excuses, or making uncaring comments such as "Well, I don't know what to tell you."
To close, this message to all pushing for a union: You'll have a much better chance of better relations from the customers AND management if you put 100% effort in doing your job. Do everything you can to make the customer feel welcome, and that includes being pleasant, and delivering the absolute best service you can. And act on the supervisor's or manager's instruction as soon as is humanly possible. Recently at a pizza buffet, I heard a manager asking a kitchen employee "Did you make that apple/cinnamon pizza [dessert] like I asked you to?" The kid simply said "No." Ten minutes later, the old cinnamon pizza with one piece left was still on the buffet. A few weeks earlier at this same place, one staffer was just playing with the pizza dough, going at a snail's pace, and the staff was just rearranging the 10 to 20-minute old buffet contents. And again, I have witnessed fast food employees at other places go real slow.
"But Mike, they're young!" Stop making that excuse, and get to work on developing them! Teaching them responsibility! That's all!