The API allowed you to see balance, transaction history, and make payments.
Before long, the API got quite popular, and lots of people were using it to make lots of payments automatically.
You know what makes money laundering super easy... a banking API so you can split the million dollars you want to launder into 1 million 1 dollar payments. And an API client which lets you treat each login cookie and account as an object in python.
Before long, the API and all users who had ever used it were perma-banned from the bank, because it was determined they posed too high a fraud/money-laundering risk.
I'm still salty because, after spending a week of my life building and refining that API, they banned me and 6 years later still claim they can't return the $350 that was in the account because they are still 'investigating'.