An intermediary between Valve and Mastercard likely was the one that brought it up because they have to comply with those rules, or the rules of someone upstream of them that has to and imports them into their own rules, so they have to interpret vague "brand damage" rules and they err to the conservative side because if they run afoul of the rules they could lose access to process Mastercard transactions ~20% of US transactions, which would really mean losing most of their customers not just the 20% of Mastercard flows.