You are right about American freeways. City roads tend to have a lower speed limit that is probably appropriate and it would be unsafe to go faster.
Rural America will have little roads with deer and intersections at blind curves and say 55 mph seems about right.

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.