My mom had one of those awful things. It strikes me that the cost-benefit analysis necessarily relied on wildly speculative premises (such as how many Pintos would actually be rear-ended with sufficient force to make them explode, etc.) and thus was utterly bogus as a calculation. As a legal fig-leaf, however, it probably seemed like a good idea at the time, but they didn't figure on how heartless and calculating it would make them look when Mother Jones magazine leaked the memos (another unknown unknown).
We're discussing some obvious limitations of cost-benefit, i.e., utilitarian, thinking in ethics.