A sound system is not necessary for blasting whatever music through a 10-50 Watt amp or more. The reason such loud soundsystems are allowed is because, so far, no data correlating accidents to using loud stereos has been tabulated. Remember, it took well over a decade to get cellphone use in cars regulated and/or made illegal.
And if a soundsystem is necessary for information purposes, a lot of that information is via the AM band -- even TIS's like the ones at airports or mountain passes, because the FM band is full. So, in my view, the Federal Government has every right reason to mandate AM in cars.