Very simple. You are only required to purchase auto insurance if you want to drive an automobile. It is your choice to buy a car or take the city bus or just hoof it. If the government said "You MUST buy a car and you MUST buy insurance" then it would be the same thing.
you cant be required to buy car insurance, even if you have a car, as long as you do not drive on public roads. i dont have car insurance, i spend that money on metrocards and i ride the subway. this is what i like to call "freedom of choice".
Shane and martin are correct but to distinguish it even more, think of this. You're required to carry liability insurance because your actions cause damage to others that must be paid for. So your insurance effectively covers others and you can add collision insurance to cover yourself which isn't required (in LA anyway)
If you want to register the car (get a license plate), you must show proof of liability insurance. If you want to use the car as a planter in your backyard, you don't need insurance.
See a trend? Get away from the bastions of liberalism and talk to lawyers who actually argue cases before the Supreme Court.
The beauty of this is that we will eventually get an answer and will know who was right. There are not enough liberals (who disregard the law and rule based upon their idealogy) on the SC to give Obama his little jewel here.