"Visa holds a 60% share of the credit and debit card market, followed by Mastercard with 30%, according to Ellis, with American Express far behind at 8.5%."
That puts Discover + AmEx at 10% or so.
In reality an intermediary called “payment processor” handle all CC brands in single system and they are the ones enforcing ideological punishments to merchants.
Like there’s web search and web browser, card networks and payment processors both should be brought up.
Yes, payment processors enforce the rules, but don't typically make the rules.
As I am sure you can imagine, this can result in plausible-deniability-type finger pointing ("we don't make the rules" / "we're not responsible for the rules being misapplied") when an organization is cut off.