In the US, at least, if the supervisors pursued legal action, they'd have a case if their job duties were closer to those of non-exempt employees than exempt ones.
Also, this is why labor still organizes in the US, even if organization is down substantially over what it used to be.
Unfortunately, the state that this occurred in has considerably defanged unions. There is one for the workers of this company, and its a subset of a national union, but they have no power and the company merely humors them just for good show.
And yes, if the workers had pursued legal action, I think they would have had a case, but these people were somewhere around lower middle class or poverty level and they were very afraid of losing their jobs. Knowing them, and others who are in similar situations, I completely understand and sympathize. When you're treated like crap, but given the illusion that you're respected and needed, you'll convince yourself that everything is "good enough." I myself did it for many years.
Also, this is why labor still organizes in the US, even if organization is down substantially over what it used to be.