That's great for cars built before the regulation were put into place. Without that regulation, you'd then be dependent on the end user purchasing an after market part and installing it. The vast majority of them won't. So if it is so important to have, you make it part of the car. They did not leave seat belts up to the owners to install after market versions.
My point is that if a 3rd party manufacturer can produce and sell a combination screen and camera for $20 for a profit, an automotive manufacturer has no reason to complain about the "expense" of such a setup. It is even cheaper for them than a 3rd party addon supplier since they buy in larger bulk and can integrate mounts for those devices into the car, rather than trying to devise some sort of one-size-fits-all mounting system that the addon manufacturers need.
They might as well be complaining about the costs of a rear view mirror, it is nonsense from the start. If a $20 gadget breaks the bank on a $30,000 minimum vehicle, they are a shitty business to start with and we should all be clapping our hands when they go out of business.
The 3rd party guy isn't paying someone $40/hour to install the $20 unit. The $20 unit will not be as integrated into the car and will have the look of an after market part. Does the $20 part only come on when the car is in reverse, or is it on all the time? There's a lot of reasons the after market thing can be $20 and a lot of reasons the auto manufacturer's is not. It's not all down to greed