I am in CT and DH was in a fender bender with two seats in the car, a graco turbo booster and a britax marathon, The marathon did not need to be replaced as it passed the guidelines, and britax goes by that.. but the turbobooster did need to be replaced, what ended up happening though was that there was so little damage to our car and if we went ahead and fixed it and had them replace the turbobooster, we would have to pay the 500 dollar deductable, and honestly a few scratches on the fender and a turbo booster really were not worth that.. so we replaced the turbo out of pocket and wiped the scratches off with a mr clean magic eraser (which works btw) That was that.. the other driver was at fault I guess.. she was supposed to have been.
I do not think there is a law, I think that they are supposed to replace the seat if the manufacturer says to weather they want to or not. you pay them
you are the customer, you need to get what you pay for IMO