Whether or not all law enforcement is racist, the system they abide by is racist. The police exist because they are supposed to keep the people safe from crime. Yet, if one were to look closely at what law enforcement does, a different story is told. For centuries now, the police have done their job, maintaining law and order, but this is at the cost of allowing change to take place. The institution also has a hand in specifically abusing people of color, whose earliest forms of policing took the form of slave patrols in the 1700s 
How policing is done also contains racial bias. Often, the police patrol neighborhoods that are predominantly poor, neighborhoods whose residents are, for the most part, black. As a result of a more prominent police presence, any arrests made impacted black people the most. 
In this situation, placement is crucial because it predetermines what races are being directly affected by policing. A large part of why this occurs is because of something called implicit bias, which is the act of harboring predisposed prejudices. 
Since racism in and of itself is a set of predisposed prejudices, the decision to focus policing on these low-income communities aptly reflects law enforcement's own implicit bias.