It's pretty much codified. 9 mph over means pretty much nothing. If you are a little over that? Everyone knows the courts will probably knock it down to 9 over just by asking.
As another comment points out, it's works for being selectively enforced. Almost every car on the road can be pulled over. Even if you are going the speed limit, you might get pulled over for impeding traffic.

One near me does it. They just happened to have a special where a month cost less than a single wash and my spouse went for it.
Nightmare to cancel the damn thing.