Yes, fast food workers are mistreated. They are underpaid for their work and many times under-trained. They are in a service position that requires pleasing customers and taking a lot of flak from them as well as from their bosses. As a result, they often under-perform at their jobs and do not serve their customers well.
Yes, people who work in fast food are treated poorly. They are paid very low wages while working for hours on their feet. They also often have no benefits, such as health insurance, through their jobs. Fast food workers also have a high risk of suffering from burn and other injuries while working on the job, and they are often discouraged from applying for workers' compensation or seeking medical treatment when they are injured at work.
The role of fast food workers is at the bottom of the job world, where the pay and benefits are not acceptable for a basic standard for living. Most fast food workers are part time or hold more than one job to account for their basic needs. It is also a hardship for the industry due to worker turnover.
Fast food workers are not mistreated just because many look down on their industry. Fast food workers are paid for their work by what the market deems as being fair. Sometimes customers can be rude to restaurant workers, and yes, there can be hateful managers to work for as well. Overall, fast food workers are treated fairly at work.