Your concern is valid and it's not lost on manufacturers. I am familiar with one manufacturer whose vehicles will have a bus level firewall in some new vehicles released later this year. Of course, eventually all vehicles will have it. Vehicle development cycles are usually multiple years so it takes time to roll this stuff out.

You might say an internet connected head unit should just be disconnected from the vehicle bus, and there's some merit to that. But, customers want to be able to control the volume from their steering wheel or the head unit should control the subwoofer amp in the back of the vehicle, or change vehicle settings from the head unit display... so the head unit needs to be on the bus to do all of these things.

The goal is to limit what the head unit has access to on the bus. For example, there's no reason the PCM (powertrain control module) should be allowed to receive anything other than high level configuration information from the head unit (switching between standard and sport mode driving profiles for example).