If your water company stops delivering water to you because you post furry porn it becomes a problem. The problem is when a company becomes so big that it is a natural monopoly (or effectively part of a cartel), while basically being necessary infrastructure to operate in a modern society.

These payment networks fit that description, and should either be broken apart so that you get the necessary competition, or be regulated so that they have to provide service for legal goods and services in a given jurisdiction.