This law seems to be widely unknown. The resulting confusion is very dangerous to pedestrians and cyclists. I know, they seem to be a nuisance. Speed has a tremendous impact on safety. This impact is often underestimated.
Speed limits determine a democratically agreed level of acceptable risk.
Again, look at the big picture. The faster cars go, the more space they need. Faster cars pose a higher safety problem, therefore we need more traffic lights. Or people try to escape the noise by settling in the open country, causing more traffic - a vicious sycle. Keep that in mind when you see a speed limit sign.