It does to them.
A seat belt in a car is expected to perform to a certain level. If they sold something the did not meet that level then they can be held liable.
Liability requires them to sell items that meet or exceed the standards for a seat belt. If not then they can be sued.
Of course, the problem is when you put them in your car. You need to do something that will actually hold. If someone were injured in your car wearing seatbelts you will be held to a standard. That standard is the person should expect the belts to hold them in the car like a modern belt.
So installing belts with the idea that it only needs to sort of hold someone in for a light accident can be costly to you. I would wonder, if you install seatbelts in your A if you should also have a plaque saying the belts are not likely to hold and are only for decoration.
If you sell an A with belts it might be wise to remove the belts. You could be still sued if they did not work and you installed them.
