In the vast majority of the US, being able to safely back up _is_ required for the safe operation of a vehicle. I recall my drivers license test included a driving-in-reverse section. You will also frequently need to be able to back out of a parking spot, or a driveway. Even if its not critical, its as close to critical as can be. When your rear view mirror and your side mirrors combined cannot give you enough visibility to safely back up, a rear-view camera quickly becomes critical.