Friend, I think I see your concern, and I may have an answer. Most of the bureaucracy is apolitical. However, the heads and higher-ups of each agency are appointed by the currently in-office politicians.

So the upper management is composed of political appointments. And like any other organization, the upper management has considerable discretion in setting priorities.

re: "politically selective law enforcement" is not a good thing, because laws are one of the things that are supposed to constrain politicians.