Fraud? Abuse? Fine, let me put cash onto a card and if that card gets stolen, oh well, my loss. Mastercard should have no say in what what speech is considered acceptable outside of their offices. We don't care what execs at a water company think? Why do we care about the people at Mastercard?
Which is why some merchants get effectively blacklisted if they have too many fraudulent transactions
Though i agree with the idea of a debit card that doesn't allow chargebacks, but without so many annoying restrictions.
That point is not the problem though. They could just pressure Valve to refuse credit cards for all or some games. The financial aspect simply does not make sense, regardless of how you look at it (and many people had different takes in this thread).
The only angles that make sense are an ideological crusade and the risk of being sued. The first is unacceptable and the second is an utter failure on the part of the legal system.