Mastercard's rule is that they can ban whatever they want, whenever they want.
> A Merchant must not submit to its Acquirer, and a Customer must not submit to the Interchange System, any Transaction that is illegal, or in the sole discretion of the Corporation, may damage the goodwill of the Corporation or reflect negatively on the Marks.
Create amorphous rules, enforce them as you feel like, then blame others for breaking the rules...
This is due to under-regulation of these oligopolies and lack of non-profit interoperable alternatives. Some things are too important to be left to the fantastical "invisible hand."