Earlier quoted context omitted.
> was Mastercard's rule, but any one of the companies in the payment network could have brought it up to Valve Did Mastercard threaten Valve? Or did Valve precomply?
They cite a rule about Mastercard brand damage. If Mastercard didn't specify that such content would result in MC brand damage why would they cite it rather than their own rules?
a) they are worried Mastercard might randomly decide it does and punish them
b) it's convenient to be able to blame someone else
c) someone somewhere said something and the rest of the orgs isn't aware or over-interpreted a statement
Vague rules like this are great to dilute responsibility. It can both be true that Mastercard didn't tell the payment processors to force the issue and that the payment processors strongly thought they had to.