It may be worth confirming the details with your bank. Certainly for purchase transactions it is Visa/MC/AE that set the exchange rates, and yes, for ATM withdrawals the ATM owner sets the fee (your card issuer may absorb those fees for you). As far as the exchange rate for ATM withdrawals is concerned, my understanding is that it is your bank that determines the exchange rate you are paid, not the ATM owner. (I have no idea whether a bank in the US would have as competitive rate on CZK as it does for CHF or EUR.)