The issue of discriminating who you sell to in the United States seems to be premised on the fact that businesses are public accomodations according to this article:
Whether you post a sign or not, businesses never have the right to refuse or turn away customers because of their race, gender, age, nationality or religion. In addition to the Civil Rights Act of 1964, several states have their own civil rights legislation designed to prevent discrimination. The Americans with Disabilities Act also prohibits discrimination in public accommodations, making it illegal to refuse service to individuals who are disabled or handicapped.
Given that a Mixed Economy with a preference toward a free market is the economic system of the United States, from a free market point of view, this seems unnecessary to do. There will always be businesses and choice in the market. Businesses that discriminate will put themselves at a competitive disadvantage due to alienating a certain employer base at an extra cost to the business:
Businesses that discriminate based on a host of job-irrelevant characteristics, including race, ethnicity, gender, age, disability, and sexual orientation and gender identity put themselves at a competitive disadvantage compared to businesses that evaluate individuals based solely on their qualifications and capacity to contribute.
According to this Free Market point of view, using government enforcement seems to create a control where there does not need to be one.
Why does the government need to institute a legal policy to prevent selling discrimination?