They think they know. Many times, they're right too, but sometimes they're wrong. That's why evidence and due process are important.
This doesn’t work when it’s something basic like payment processing. It’s the same reason we decided as a society that people can’t “trust their gut” and deny housing to people based on the color of their skin.
Running a profitable online business is not even a basic human right. No one owes you payment processing. Forcing someone to be a supplier to a hate group violates their basic human rights. If payment processors were discriminating against a protected class such as race or age then you might have a point.