For most of American history, charging someone 20% interest on a loan was illegal. Every state had usury laws capping rates at 6%, 8%, sometimes 10%. These protections dated back thousands of years — from the Code of Hammurabi to colonial America. Then in 1978, one Supreme Court ruling quietly changed everything. The case was Marquette National Bank v. First of Omaha Service Corporation, and almost nobody noticed when it was decided. But it opened a legal loophole that allowed banks to "export" interest rates from their home state to customers nationwide. What followed was a chain reaction: a desperate...