There are labor and workers unions that protect the rights of workers. If full-time workers are being paid less than $20,000 a year, those workers would either leave the company or form a union. However, you need to look at the flip side of this. The employees being paid under $20,000 a year are most likely part-time employees. Companies cannot afford to pay their workers more than minimum wage or more than their work is worth. Therefore, due to various circumstances, it is ethical to buy goods from companies that pay their workers less than $20,000 a year.
According to the National Institute of Health $20k/year seems to be the benchmark for people in the US to meet their hierarchy of needs. Below that benchmark human development is significantly stunted and mental illness sharpy rises creating great suffering for laborers. Low wages are only kept low so dividends can be paid to the very few investors within our society. For many to suffer and few to gain a low level of pleasure, no matter how you spin it, every time you purchase goods from a company paying employees under $20k a year you are acting unethically.