That's just standard separation of concerns, expecting individual police officers to make judgments about each case would make law enforcement very inconsistent.
People who are skilled at making arrests and beating down doors are not necessarily also skilled in analyzing data and crime statistics in order to decide which cases to persue.
Prosecution seems to be politicised, and pursue cases according to who controls the executive branch rather than according to the interests of the general public. I'd like to know which countries have taken steps to make prosecution more independent, and how they did it.
Sure they do.
In some countries (e.g. UK) the prosecution service (e.g. Crown Prosecution Service) only bring a crime to court if it's "in the public interest" (cf. http://www.cps.gov.uk/publications/code_for_crown_prosecutor... ). Ireland has similar laws ( http://www.dppireland.ie/brief-guide-to-the-criminal-justice... )
Although this applied to the actuall criminal prosecution, not the police enforcement.