If KiwiFarms hosted/was responsible for illegal activities, then it is up to law enforcement to deal with it, not payment processors.
It is not desirable that private infrastructure companies such as Visa or MasterCard are effectively playing as judge, jury and executioner in modern society, even if (sometimes) they are happening to do the right thing.
OTOH, plenty of sites have been canceled simply due to their fucked-up viewpoints that are protected speech.