I think legally they would need to require using an app for their back view camera. All new cars in the United States after 2018 need one and I don't see how it would work without using the phone/tablet as a display.
Sure, ultrasonic sensors have a better outcome than backup cameras in a cost-benefit analysis (https://www.nhtsa.gov/sites/nhtsa.gov/files/nhtsa-2006-25579...) and allow you to simultaneously look out the windows and get feedback from the sensor, but allowing them would have allowed for the sale of cars too cheap to have infotainment systems, and that was not the goal of the regulators.