The sale of cannabis is legal in 24 US states, and another 15 states permit sales for medical uses. However, the federal government considers all sales to be illegal. Consequently, all credit and debit card networks in the US refuse to process transactions from cannabis dispensaries, where sales exceed $40 billion annually. Almost all dispensaries have ATMs on the premises to let buyers obtain cash for a payment. Some dispensaries operate outside network rules and allow payments to be made by credit and debit cards by falsifying a merchant category code in a process called “cashless...