Earlier quoted context omitted.
Being young means you're less likely to be totally responsible yet. Being poor means you're less likely to have parents that are 100% engaged in your library visits.
Small fines seem like a great way to instill a sense of responsibility (while also acting as insurance style payments), especially if your parents already aren't engaged and ensuring you're responsible to duties. As this the Good Will Hunting quote suggests, in many libraries these fees were small, $1.25 in late fees for a Harvard Education...
So it actually may have actually been detrimental.