Do you think 93% of the white people aren't working/lower? We're talking about a crazy arrest rate. Insane if you think about it.
That isn't the point (and I honestly don't know the amount). 2/3 of the population in Ferguson is African American. And African Americans population have a larger percentage of working/lower class than Caucasians.
Other facts that lead to higher crime rate among African Americans:
African American and Latinos high school drop out rates are significantly higher than Caucasians
African American children are three times more likely to live in poverty than Caucasian children
Unemployment rates for African Americans are typically double those of Caucasian Americans
African Americans and Latinos are more likely to attend high-poverty schools than Asian Americans and Caucasians
African American men working full time earn 72 percent of the average earnings of comparable Caucasian men and 85 percent of the earnings of Caucasian women
Crime has a direct correlation with low income, low education communities.
It's the hard truth and the cause of many factors outside their control. But that's the nature of the beast.