According to the Department of Labor, all employees must be paid for time worked. A common myth is that if you are a “salaried” employee, you can be forced to work 40+ hours and not be paid for any hours over 40. This is simply not true and a common scam among employers or ignorant business owners to get out of paying employees. An employer must either pay you, or provide you time off for working over 40 hours. Any hours over 40 is considered overtime. Your first step should be reporting this to the Department of Labor since it is a federal agency, and they take investigations seriously. Employers will get away with whatever they can as long as no one questions or reports them.
This is a story recently in the news that demonstrates that employers do not understand they are legally required to pay overtime.
Many people eventually choose to quit due to not being paid for working overtime. In this case, the quitting would be for good cause since a reasonable person would not put up with working for free.