You've heard that a company can't fire a worker in retaliation, and that's true. However, it only applies when that retaliation was for an activity that is legally protected.
So, what areas are protected? Here are two examples:
The worker is involved in an investigation
For instance, investigators think that wage and hour violations are happening within a company. Tipped workers are not being paid properly. The investigators come in and begin questioning employees. One worker gets out of an interview and is promptly fired by an angry company owner -- who perhaps knew all along that the violations were taking place. That's an illegal firing. Workers are protected when they work with legal investigators.
The worker reports an unlawful practice
For example, truck drivers are only supposed to work a specific amount of hours in a row. However, to get more done, the company tells the drivers they have to work extra hours and then lie in their log books. One of the drivers reports the behavior, knowing it creates a dangerous workplace environment and increases the odds of a deadly accident, and is then fired for filing the report. Again, this is an illegal firing.
Workers have rights. They have legal protections. While companies may be in charge of those workers in a general sense, they can't control all of the actions that they take and they can't fire workers based on protected actions.
Those who have been fired under these types of circumstances -- and the above are just two examples -- absolutely need to know all of the legal options that they have moving forward.
Source: FindLaw, "Retaliation and Wrongful Termination," accessed Feb. 02, 2018