If someone wants to steal a book, why would they come back and pay the fine? The only people who would pay the fine are the ones who want to continue checking out books, so preventing them from doing so if they have a late book is incentive enough.

This is a pragmatic move by the libraries. The reality of fines is that they don't prevent bad actors from stealing books. All they accomplish is disincentivizing those who miss their due date (whether intentionally or not) from bringing the book back.

Chicago did a similar thing a couple years back and actually saw an increase in returns.

https://chicago.suntimes.com/news/2019/10/30/20940677/chicag...