Just asked Bard why it was legal for banks to use customer deposits to generate yield, and here are some nice blurbs from the response:

"It is legal for banks to use consumer deposits for bank yield because it is a necessary part of the banking system. Banks need to generate income in order to cover their costs and generate profits. Bank yield is one of the primary ways that banks generate income."

"Without bank yield, banks would not be able to lend money to businesses and consumers. This would have a negative impact on the economy. Additionally, without bank yield, banks would not be able to generate profits for their shareholders. This would make it difficult for banks to attract capital, which could destabilize the banking system."

"Overall, there are good reasons why it is legal for banks to use consumer deposits for bank yield. Bank yield is essential for a healthy and stable banking system, and it is protected by government regulations and FDIC insurance."

The 'essential for a healthy and stable banking system' is classic