America's largest banks had a myriad of worries in mind when they rushed this week to ban customers from using credit cards to buy cryptocurrencies. Bitcoin's gut-wrenching slide was just one of the threats.

JPMorgan Chase & Co., Bank of America Corp. and Citigroup Inc. started to decline purchases as industry executives zeroed in on a variety of ways they could get burned, according to people briefed on the decisions. Publicly, JPMorgan cited the risk that borrowers might not repay. Behind the scenes, card issuers were also concerned about the protections they offer shoppers and their vulnerability to thieves, the people said.

Near the top of many lists were initial coin offerings. Startups have embraced the fundraising method, selling tradable tokens to gather money for projects, sometimes promising future rewards. ICOs drew $3.7 billion last year, but in many cases companies have struggled to make good on obligations or revealed themselves as scams. Increasingly, regulators are intervening, deeming some tokens to be unregistered securities.