Because this is America and there is a continuous history of racial bias since the country was founded. Or did you think that all went away with the 13th amendment? Or the 14th? Or maybe the 15th?

The Department of Justice has reports detailing racist enforcement in Ferguson http://www.nytimes.com/2015/03/04/us/justice-department-find... and Newark http://time.com/3020180/newark-police/ and the New York Bar Association had a report on stop-and-frisk being a racist policy in NYC http://www2.nycbar.org/pdf/report/uploads/20072495-StopFrisk... [PDF] and a St. Louis police union is calling out racism in the department http://www.nydailynews.com/news/national/king-black-union-st...