TL;DR: Automakers want to delete AM receivers because reduced receiver performance in electric cars will drive up service returns and reduce perceived quality.
I designed the IF downconverter chip for the first generation of HD radio in North America (first generation had separate chips for RF, IF, and baseband processing). I was witness to some of the issues raised by automakers with regards to HD radio implementation. One of the most onerous was driven by fear of increased service complaints / warranty claims (added cost was another issue, but that was at least partially solved by subsidies from the National Association of Broadcasters (NAB) who were terrified that the new satellite radio would drive local radio stations out of business and pushed HD radio to compete).
Originally the system was designed to provide CD-quality digital audio out to the 1mV/m field strength contour which the FCC uses to define the minimum required range of an FM station (“The transmitting location should be selected so that the 1 mV/m contour encompasses the urban population within the area to be served.”) Once below that signal level the necessary bit-error rate (BER) to sustain digital audio would not be guaranteed due to noise and interferers.
That was not acceptable to the automakers. Their argument was that customers were used to getting acceptable FM reception far past the 1mV/m contour with conventional analog receivers. Any digital solution that started dropping in areas where the customer was used to getting reception would result in the car being brought back - “The radio’s broken.” They insisted that (1) the digital audio would fail incrementally with decreasing signal strength and (2) reception in all cases will be at least on par with the existing radios.
For those of you interested in the nitty-gritty: these requirements led to splitting the digital audio stream into four channels with lots of error correction; the adoption of the Lucent Perceptual Audio Coder (PAC) which can encode the digital audio into the four channels in such a way that loss/error in up to three of the four channels would still allow the audio to be decoded (at reduced resolution); and synchronization of the simulcast analog broadcast and the digital broadcast to allow a seamless transition between the two when the digital craps out completely.
This drove up the cost of HD radio substantially, but the automakers are terrified of anything that drives up service visits and even hints at a reduction in quality.
If indeed AM radio reception is problematic in electric vehicles, I can see the desire for the automakers to drop it.