LSB's different prices for the same thing!!

Banks get charged for currency exchange. Why? I have no earthly idea - it's 100% digital. It's not like some poor sap gets sent to a local bank with a briefcased full of euro's, hands it to a teller who has to count it, run it through a conversion calc, then count out the amount of dollars it's equal to, then chage the customer a fee for the effort. It takes a millisecond (probably less) for a computer to figure it out, yet, people get charged for it. Why?