Similar to the recent actions by Twitter, Google, Apple and Amazon; Visa and Mastercard are private companies and can do whatever they want. Anyone who has an issue with what Visa/MC did must also take issue with the others.
This rhetorical trick of categorizing all of these as "private companies denying service," is the same one used by those who opposed anti-discrimination laws. Should an ice-cream store be able to kick out a person pissing on the floor? Should they be able to kick out a black person for being black? In each case, it's simply "a private company doing whatever they want." But, obviously, these two cases are not the same.
Personally, I think it's quite a different thing to kick people who incite violence off your social media platform than it is to ban sex-workers from using you as a payment processor. And, because they are different actions, with different intent and effects, it is, of course, possible to think "Hey, it's good that [x] is happening, but bad that [y] is happening."