Selective enforcement is not a good outcome. Any police officer who doesn't enforce a law because they don't like it should be fired.
Not saying it's a good outcome, but in my lifetime, that's what I've learned to expect. If the chief doesn't like the law either, who is going to fire the officer?
The police don't sentence people to jail.
No, but they do pick who they want to pull over, and for what reason.
That's their job?
They de facto do. A police officer can arrest you for basically any reason and send you to jail. How long that jail stint lasts depends on what happens after you are arraigned, but you can easily spend a few days in a county jail before the judge shows up after the 3 day weekend to decide if what you were arrested for was legit.