The main reason why you will alway have a low voltage battery is safety: It is used to actuate the contactors for the HV Battery. You would not want un-switchable HV components. For example in the event of a crash the HV battery gets completely disconnected but airbags and indicators will still work. The 12v battery will get charged off the main hv battery if it gets low btw.
This is not at all a LED problem. It is just very poor execution. When we switched to LED in our and neighboring cities the light pollution went down very noticably. The switch should be used to use light smarter, not just blast more with higher efficiency. We now have some streets that even have motion sensing and dim to 5% power when not in use.