It seems unlikely that the connection between fines and large African American populations—a connection that cannot be explained by poverty—is the result of African Americans across the United States committing more finable offenses.
A more likely explanation is that they are more highly policed and that, as has been acknowledged by the Director of the FBI, African Americans suffer from heavier legal penalties due to the implicit bias of police officers.
Charging individuals with minor infractions is highly discretionary and influenced by where law enforcement choose to direct their attention. Although African Americans and Whites report smoking marijuana at the same rates, African Americans are 3.7 times more likely get arrested for marijuana possession.
Other research shows that while African Americans are less likely to sell drugs than Whites, they are more likely to be arrested for it. An analysis of the National Survey on Drug Use and Health found that 6.6% of White people between the ages of 12 and 25 have sold drugs compared to 5.0% of Blacks. Yet Black people are 3.6 times more likely get arrested for selling drugs.
Given this evidence, it seems likely that police officers often fine African Americans at higher rates, even if Black people do not commit more infractions.