Mastercard and Visa have rather corse information about the transactions.
They've got the credit card number, the merchant name, the time, and the total amount of the transaction.
They do not have line item level filtering of a transaction. Remember those old carbon paper credit card thingies? https://en.wikipedia.org/wiki/Credit_card_imprinter - that's all that's needed and all they get. Similarly, the credit card terminals where the merchant enters the amount, swipes the card (or reads the chip) and that's it is sufficient.
Mastercard and Visa would only be able to say "that merchant" not "that product." Filtering based on products and if it's legal there needs to be done by the merchant. Mastercard cannot check to see if someone is selling liquor to an underage customer... but if a merchant is doing that, Mastercard may drop that merchant as one of their clients.
If Itch and Valve are unable to enforce Australia specific laws on their own storefront, Mastercard and Visa can only enforce it at the "this merchant isn't allowed to transact with our network."
Mastercard can deny all transactions from Australia-billed cards to one merchant if they wish to. They are absolutely wired up for “Area of Use” internally and have this data available to their transaction approval processes. That they chose not to use it, instead pressuring merchants to remove content disliked by an Australian puritanical fringe group, is the corporate laziness I describe. Why respond with their own effort when they can just externalize the problem onto their customers, etc.
Mastercard does not have that information. Mastercard doesn't do the billing. The bank does the billing.
Mastercard does not know the location where a given card holder is (or for that matter, any demographic information about the card holder). They know where the merchant is, but that's less useful for digital goods.
Per section 7 here — https://www.mastercard.us/content/dam/public/mastercardcom/n... — MasterCard could simply remove Australia from Itch’s Area of Use, at which point they would not be permitted to accepted MasterCard from Australian customers, which the merchant could trivially enforce by country filter on the billing address.
I suspect we’re going to find out that Stripe is unwilling to risk losing Mastercard in Australia and also unwilling to implement passthrough AoU restrictions to their sublicensees, and Mastercard isn’t willing to act against any single customer of Stripe or else they don’t profit from the “not our problem” discount rate they issue Stripe to make it their problem.