Are car seats safer than seat belts ?

Steven Levitt did a great speech at TED . He told the story of the car seats.

Turns out that car seats for kids over 2 are no more effective than seat belts.
His study took in consideration 45,000 cra crashes during 30 years, from 1975 to 2005.
So if it's so clear, and if it means that Americans are wasting $300 million a year on car seats, what's going on?
Why is it that in New York State it's a crime to put a six year old in the backseat of your car with just a seatbelt?
For more details check out this video.