Because racism is the discrimination of someone based on their ethnicity. If you are choosing one person over another due solely to their ethnicity, isn’t that discrimination? Shouldn’t people be judged not by the color of their skin? Explicitly advertising that you are selective solely based on race is racism.
If the concern is economic disadvantages, shouldn’t the selectivity be based on income and net worth instead of skin color? Maybe selling products from poor and independent farmers. A portion of every race is economically disadvantaged.
Edit: I really appreciate your response. I think you described the issue really well.