A couple allegories:
If a white guy goes by a speed trap traveling 70 in a 55, and the cop doesn't stop him, and then a black guy goes by the same speed trap traveling 60 in a 55, and the cop pulls him over and gives him a ticket, and this happens over and over and over again, that is disproportionate police persecution of blacks.
Then some schmuck comes along and says, "Statistics show that Negroes commit more traffic violations than whites!"
See how that works?
A white guy steals a loaf of bread. The cops come and they work it out with the store owner for the white guy to pay for the bread.
A black guy steals a loaf of bread. The cops come and arrest the guy and he gets probation.
The same white guy steals a loaf of bread again. The cops come and they work it out with the store owner for the white guy to pay for the bread.
The same black guy steals a loaf of bread. The cops come and arrest the guy and now the judge says, "You have been arrested for stealing bread before. I am going to throw the book at you."
Then some schmuck comes along and says, "Statistics show that Negroes end up in jail more than whites!"
See how that works?
The DOJ report indicates this is the sort of thing which has been occurring in Ferguson.
Racial discrimination investigations are done in response to the claim that this is what is going on. It is incumbent upon the government to do a self-check to discover if this is actually occurring.
If it turns out this is actually occurring, and the government releases evidence to that effect, some schmuck can be counted on to come along and say, "The Dims love crooks!"