That is a gross oversimplification.

Initial police reports inform preliminary charging and bail decisions but are hearsay and inadmissable to criminal proceedings unless the officer personally witnessed the crime. That is why follow up investigations are conducted. In the case of a misdemeanor, this is entirely handled by the prosecuting entity. In the case of a felony, this is handled by both the prosecuting entity and a police detective. A sound investigation on the part of the detective is an invaluable tool in prosecuting a defendant but all charging and plea negotiating decisions are made at the discretion of the attorney in accordance with the law. All conviction and sentencing decisions are made at the discretion of the judge, and occasionally a jury, in accordance with the law. At all points in this process any or all of the following factors can be in play: the defendant's criminal history, the defendant's mental health history, the personal notion of justice of the prosecutor, the personal notion of justice of the judge or jury, the political climate of the city or county, the official policy of the prosecuting entity for certain crimes (e.g., when the Seattle City Attorney stopped prosecuting misdemeanor marijuana-related crimes), the budgetary concerns of the municipal, county, or state government, the cooperation or lack of cooperation of the crime victim, and so on, and so forth.

Trying to blame the police department for the workings of the entire justice system is lazy and incorrect. Additionally, if you actually care about this, conviction information is a matter of public record and you can see it whenever you want.