Yeah, drives me nuts. Some cars have really great rear visibility and should be exempt from the requirement. I understand it on SUVs and stuff though.
A bigger rear window wouldn't have let me see the toddler who'd wandered a short distance off from his parents in a car park and was standing right behind my car.
You can't see the pavement under the bumper, but you can see a kid on a tricycle right behind the car.
Personally I find rear view cameras to be quite disorienting. The view is quite distorted by the wide-angle lens and it's not clear where it can see and where it can't. Do I still have blind spots if I look at only the screen? Where are they? With a mirror I get an intuitive sense of what it can and can't see. Also, I expect the view to move when I move my head like in a mirror, which is part of what makes it so easy to get a sense of where the mirror can see. I suppose someday I'll get used to cameras, but today is not that day.
My hatchback has about the best you could get (and they don't sell it here anymore) and I still added a backup camera, it's very useful.
My little GTI with a backup camera made me an invincible parking machine.
It also enables auto-parking and other enhancements while not inhibiting anyone that wants to perform those functions manually.