It is not mandatory that backup alarms be 97 to 112 db. They only need to be "above the surrounding noise level". The loud beeping alarms were installed on most vehicles because most of them operated at loud constructions sites, so needed to be louder than that. it was easier to just buy the loud model to CYA, even if it was a delivery truck. They also don't need to 1000 Hz or to actually beep. White noise backup alarms are allowed, and in use in many delivery trucks now, and make a sound attenuated above 4000 Hz, which is much more localized and dissipates over much shorter distances. Waymo could absolutely have installed 85dB white noise alarms but chose to install 112dB beeping alarms. This is not a regulation problem.