Here's what I got back from Mastercard, following an alert posted here
Accordingly, we do not issue cards, extend credit to cardholders, determine the interest rates (if applicable) or other fees charged to cardholders by issuers, or establish the merchant discount charged by acquirers in connection with the acceptance of cards that carry our brands.
It is important to note that any charges made to a cardholder are ultimately determined by the issuing bank. Issuing banks differ in their pricing structures charged to cardholders and MasterCard does not take part or have any influence in the final decision on total charges levied onto cardholders. Therefore in this case, the percent service charge to cardholders has been determined by Dah Sing Bank.
On our part, MasterCard charges an Issuer Cross-Border Assessment on issuers for all cross-border transactions. This means that the charge applies whenever the merchant (or merchant acquiring bank) is located in a cross border location from the cardholder's issuing institution and regardless of whether the transaction currency is different from the cardholder’s billing currency or not.
MasterCard mandates that issuing institutions provide disclosure to cardholders on any amounts in respect of the MasterCard Issuer Cross-border Assessment that the issuer charges, or will charge, to the cardholder. In May 2005, MasterCard advised member financial institutions of a restructuring of existing cross-border fees. We understand that Dah Sing Bank has provided the necessary disclosure. The revised fee was only implemented in April 2006.