This is a really good point. I think payment providers are like utilities these days, as cash becomes more obscure in society.
This is the reality. Payment processing is a utility. Small businesses literally cannot operate online without payment processing. On first glance there appears to be many third party providers, but ultimately they are all processing Visa/MC, and are selecting customers on the same criteria.
We have non-discrimination laws on a variety of protected classes. It seems there could be support for expanding those classes, like the recent story about political affiliation and that dog shelter.
Many businesses can't discriminate based on set classes, but they broadly aren't required to take on all customers