Why don't people have them anyway? Whether its law or not, you want your children to be safe right?
I wouldn't let my DC go in a car without a car seat whether it was law or not. Years ago, we'd been on a day trip with the PIL and DD (then 3) wanted to travel home in their car - I went to my car to undo the carseat and when I got to their car, MIL had sat DD on a cushion - it wasn't law then, but there was no WAY she was travelling like that.