These are people who are often working for our convenience and at their severe inconvenience with little in the way of recognition. Instead, they are often treated horribly by "entitled" customers who are looking for someone that they can bully. Any inconvenience (real or imagined) to customers seems to make some that feel that they have the license to be cruel to those who they believe are of a lower social status than them.
Meanwhile, these employees are often given bizarre and shifting schedules, few (if any) sick or vacation days, mind-numbingly dull tasks, and chronic injuries due to repetitive stress tasks. Worse, they are often maligned in public realms as being stupid or lazy (and some are, but I've found that true in all professions). That they are paid less than a living wage is itself a travesty; that people publicly decry and insult them for wanting better is a sad commentary on our society.
Now, I plan on going shopping on Black Friday (although I do refuse to do so on Thanksgiving itself). When I do so, I try to cheerful, friendly, and polite. They deserve at least that much.