I hear conspiracy theories like this but I've never seen any evidence of it happening. Reason is that the cops would get in trouble.
Uh, no. I was born and raised in a small town in central Kentucky. My mother was the city attorney. One police officer failed to get an oil change or routine maintenance on his cruiser for 20,000 miles (they are allowed to take their cruisers home, but get free maintenance at the city garage). The engine on his cruiser then failed and had to be replaced. As a result, my mom set in motion the proceedings to have the officer fired. The city was unable to fire him, and from there on out he would, whenever he saw me driving around town, follow me and pull me over for minor offenses such as speeding less than 5 mph over the posted limits. Eventually, his wife got a job in another city and they moved.
The fact of the matter is that it is VERY difficult to have police fired. It used to be commonplace for mayors to come in and fire most of the police department and repopulate it with his cronies, now there are all sorts of laws protecting police officers.