You seem to be conflating law with rules of the road. The rules of the road are just how (mostly) everyone (mostly) agrees to behave (most of the time). Basically it's social consensus. The end result is similar but not the same thing as the law (which makes sense since laws reflect norms). Cars mostly follow the socially agreed upon rules of the road regardless of the law.

Cyclists and to a lesser extent scooters throw a wrench into things because we as a society haven't yet had time to reach a consensus on what the norms for these classes of traffic are and how they should behave so people fall back to the law.

This is why people get all "but the law" when cyclists run reds but don't care when cars roll stops or speed (under appropriate circumstances). We've established that the latter is ok but haven't yet reached a consensus on if the former is to be expected as part of normal traffic flow.