Find some documented case where a US police officer was punished, in any way, for pulling someone over arbitrarily. There are too many loopholes. "Crossed center line", "erradic manuevers", "tint seemed dark", "appeared to see open alcohol container", "did not come to a full and complete stop", and so on.

We even have full on Stasi style checkpoints occasionally, where everyone is pulled over.

Sometimes the violation is driving 5 miles above the speed limit on a highway.

"Driving too close to the white line."

Or my personal favorite: "Driving erratically"

Sure. Selective enforcement is a huge problem. But not giving a reason at all is nuts.