It is my understanding that men who frequent prostitutes often select on the basis of physical characteristics such as hair color and length, breast size, skin color, etc. If a john wishes to have sex only with white prostitutes or only with Asian prostitutes or only with black prostitutes--or anyone EXCEPT a white woman, Asian woman, black woman, whatever---, is that john not being a racist? Should his racism be sanctioned by being allowed to frequent only prostitutes of a certain race? How is this different than a prostitute refusing clients of a certain race?
Because the john isn't running a business - which has been the entire basis of the argument. He's making an individual choice in his personal life, so he can be as racist as he wants for whatever reasons he wants and it's nobody's concern except his own. It's no different than his having hamburgers and sushi but never going into the Ethiopian place. People in the government can be disgusted by him for being a racist asshole but they have no basis to sanction him. If one of those restaurants has an Ethiopian guy walk in and they tell him to go and eat elsewhere because his kind isn't welcome, however, then the government has a basis for the sanctions.
The prostitute is running a business and her licence to run that business is dependent on her following the rules for conducting business. If she doesn't pay her corporate taxes, the government can revoke her business licence and that doesn't make the government her pimp because she's passing on the cash she made from sex to them. If she doesn't follow health and safety rules, the government can revoke her licence and that doesn't mean they're forcing her out of the security of a brothel and out into the danger of the street. If she doesn't follow anti-discrimination rules, the government can revoke her licence and that doesn't mean they're coercing her into sex.
It's the government regulating that a legal business must comply with the legal rules of conducting business. That's irrelevant to a customer deciding which business they want to go to.