Check out the discussion here around potential child safety implications with car seats, though it seems like the curtains do come standard still - perhaps that’s why it is optional? At the very least, that means you get to fork out an extra $400 if you want it!
The majority of cars do not have rear side impact airbags, they have rear curtain airbags. No volvo in the lineup has rear side aidbags, and I don’t think majority of BMWs have them either, could be wrong. I know subarus don’t.
Didn’t realize its optional in most cars but I would simply add it to standard equipment and increase price if I were Audi. They can always highlight it as " we have better standard safety equipment" . The message I get as a customer now is " We can save your life but only if you pay $400 more". Not to mention many people would not even realize this is even an option.