Implied consent for drivers is from a bygone era when cars were not the dominant mode of transportation. Mass transit in LA (and most of the rest of the United States) is a woefully inadequate alternative. You can't consent to something when there is reasonably no other choice.

Your argument would only hold water in the future if self-driving cars become widespread and it isn't necessary to display a number plate on a car without a human driver, thereby giving people a reasonable transportation alternative that allows for anonymous freedom of movement. Until then, the government should not be able to simultaneously mandate number plates and use them for mass surveillance.