These days alot of fast food places are understaffed, and so its just easier for the customers to dismissively wave their hands and assume everyone is an idiot instead of looking closer. And in the case of fast food, it should be expected that you get what you pay for not only in terms of food quality, but also the service.
In the rare occasions I'm in a fast food place I'm usually much more annoyed with the customers, who often tend to be loud and whiney slobs, than the service staff.