To those defending retail clerks, restaruant workers, etc.
I think those people need to remember WHY they have a job. They aren't there to hang clothes on a rack, they are there to sell them to a customer. The more pleasant, cheerful and helpful they are, the more clothes they will sell. They aren't there to take the fries out of the hot cooking oil, they are there to sell those fries. The more pleasant, cheerful and helpful they are, the more likely they will sell that person a soda and maybe even a hamburger to go with it. They have a job because of the customer.
I worked at a Wendy's franchise for 6 1/2 years (3 stores, same company). You treat customers decently, they will do likewise. I can't remember how many upset customers got over their anger in a hurry when they found out that someone cared and would help them solve their problem.
It pays to treat the customer well, even if they are jerks.