Ridiculous that a few large corporations get to decide this. The only people who have any business deciding what content is or isn't allowed on someone else's platform, are the People, or their representatives in government.
I can accept that some content goes too far and might have to be banned according to some people, but the only groups who should have the actual power to do that, should be the owner of the platform, the users of the platform, and the government, and definitely not some third party that might have completely different interests.
The only reason this is possible at all is that some of these payment providers are nearly monopolists. People should have about a dozen payment options to avoid this. Fortunately I do; Steam supports iDeal, which is not controlled by any payment provider. It just goes straight to my bank. If my bank were to try to ban certain types of legitimate payment, customers could simply switch to a different bank to avoid the ban.
The only kinds of payment that payment providers should ban, are those involving financial crimes, like money laundering, fraud and corruption.