If the data itself is bad and known to be bad, the results will reflect the bad data.

In the case of policing, the arrest record is racially biased (the US releases data that shows this bias and it's well researched). If you train on arrest records, it won't predict actual crime because the results will be racially biased.

Being unbiased is actually a very difficult problem. Especially when we are predicting things rooted in social / cultural trends.