You're mixing several (valid!) concerns here that I think deserve being addressed individually.
Visa and Mastercard are card brands/schemes/networks, but definitely not issuers of credit cards, and accordingly also not issuers of credit. No fiscal policy is happening here; if anywhere, you'd have to look at banks for that (and it might be interesting, but in my view this has nothing to do with cards).
What they do control to a concerning degree is the ability of merchants to accept payments in the first place, together with acquiring banks. If Visa and Mastercard decide that merchant category x or charity y can't accept payments anymore, that's a big problem.
In my view, this is a task best left to democratically elected governments held accountable through checks and balances, not to private corporations. Of course, these will also have to follow regulations – but sometimes, they go above and beyond (e.g. out of brand recognition concerns), and in doing so end up exporting decisions and policies into distant corners of the global economy that have no political, legal, or economic recourse.
Finally, you've got the concern of market dominance and whether there is effective competition in all relevant markets (e.g. merchant-side in addition to just issuing-bank-side) between the two, and whether that leads to inefficient prices for merchants. But as I've mentioned elsewhere, the biggest chunk for card processing fees is paid back to the cardholders as points in the US, not to Visa or Mastercard.