1% is charged by the card association. 1.8%-2.5% is charged by the bank.I doubt that's the fee or the spread the card associations charge the banks. The banks can choose to clear and settle in charged currency. Can the card associations force the banks to convert to its local currency? I doubt it, maybe only for currencies not easily obtained. The banks are not carrot heads like consumers.
I believe forex is performed by the card association. For example, Mastercard has a calculator that gives a good indication on the rate they use every day: https://www.mastercard.us/en-us/personal/get-support/convert-currency.html
Isn't this the premise behind multi-currency debit cards like DBS Visa Debit, UOB Mighty FX, and HSBC EGA?imagine a transaction that Amazon charges USD to a DBS credit card. DBS certainly can obtain market rate for USD to settle the transaction. Can Visa/MasterCard force to convert to SGD with spread and charge DBS? I doubt DBS is that weak.
It's probably doable for credit cards too, since ICBC has a dual currency card: https://singapore.icbc.com.cn/ICBC/...CardProducts/ICBCVISADualCurrrencyCreditCard/