The main evidence is to take these companies at their word instead of assuming they're trying to cover something up. They're willing to allow payments for 90% of fetishes but exclude the other 10%. Are they really drawing the line based on the rates of fraud in each fetish?
Cases like the FetLife are illustrative. You can also look into f-list, a furry website which ultimately had to decide not to take any payments when every major processor rejected them. The key point is that companies are required to distinguish between "pornographic" and "obscene" content using a vague case-by-case smell test. So you sit down with a lawyer and kinda guess what's OK and what isn't and cross your fingers the credit card companies swallow it. That's exactly how this story was resolved. You can learn more about the law yourself by looking at an example ToS from Square and some guides to federal obscenity law.
https://www.justice.gov/criminal-ceos/citizens-guide-us-fede...