The GP also seems to think that having rigid car seats on plane would be a net new thing. They are already allowed and flight crew already check that it has the right certification and is attached correctly - this is much easier on a plane because the only way to attach them is the seat belt versus most seats have at least 3 different ways to attach in a car.
What do you think people are attaching the already approved rigid car seats to the aircraft with?
The argument, quoted by the GP, is about improperly restrained seats. They are only talking about those.
The GP also seems to think that having rigid car seats on plane would be a net new thing. They are already allowed and flight crew already check that it has the right certification and is attached correctly - this is much easier on a plane because the only way to attach them is the seat belt versus most seats have at least 3 different ways to attach in a car.
You're right, I forgot that's impossible with a carrier seat.