I'm going to disagree with this. A lot of business services are near monopolies just by virtue of geography, especially if you live at all in a rural area. There are places where there is one decent supermarket, one... whatever. Other companies serve very unique niches in business environments.
It's all well and good to say that if a company has a bigoted service policy, customers can take their custom elsewhere, and capitalism will reward the creation of an alternative. But there are places where exclusion from one or a few businesses can create a great personal hardship and make it difficult for excluded parties to pursue their own careers.
Remember that we're not that long past the decades of real segregation, where businesses making exactly these kinds of decisions had a real and brutal impact on the opportunities available to black people in the US.
Now you may say "Oh, we're past that now though, any company that tried that would be committing suicide" But only last year, an entire town conspired to keep one girl out of the prom because she was gay. They went to the trouble of organizing a decoy prom. This is a whole freaking town. The only reason they have any repercussions, and the only reason this is national news is because there is a legal structure in place to prevent such actions when schools are involved.