Just to clarify Milton Friedman (A champion of free market economics) readily acknowledges racism. His argument is more along the lines of "in the long term racist businesses will lose business to more accepting and diverse ones."
Without the federal crackdown on discrimination in the 1950's and 1960's, housing, employment, public places, education, marriage, etc, would have remained segregated. In the background of that segregation, how long do you think it would have taken to get from "typical American circa 1950" to "typical American circa 2014?" Laws create social norms, and laws banning discrimination allowed a couple of generations of Americans to grow up in an at least somewhat integrated world, which led to the America of today where you can see people boycotting a business for serving blacks as something difficult to believe.
It's sort of cute in the abstract, but it ignores the reality of people suffering now.
Just another point against Friendman and his "understanding" of the world.
This assumes that the business differential exists and is large enough to matter.
I generally point out the lack of non-smoking bars/nightclubs before all the anti-smoking laws went on the books. Everybody agreed that non-smoking venues were a good thing, but non-smoking bars/nightclubs simply did not exist because the business differential was too high (smokers were so much more profitable than non-smokers that even though non-smokers outnumbered them nothing would cater to the non-smokers).
In the case of AirBnB, how much profit will someone give up by not serving <ethnic group>? Not much since most of these places are just getting a little bit of profit for renting out extra space.
For example, if 5% of blue people trash your house, and you can't distinguish the 5% from the 95% easily, it may be rational/profitable to discriminate against blue people.