I think you are confusing security and safety. Security is about dealing with malicious attackers, while safety is about making sure random events and mistakes won't kill you.
Your example with the brakes is about safety, i.e. making sure the car won't kill you during normal operation. Normally, unless their CPUs start bursting into flames, this is not a problem for Intel.
The problem here is about security. A car analogy would be that to start your car, you need a code and that code can be found by measuring how long it takes to process the input, making life easier for thieves.
As for liability, I don't think you can be liable in court if you didn't plan for something that wasn't known at the time and isn't trivial.
Your example with the brakes is about safety, i.e. making sure the car won't kill you during normal operation. Normally, unless their CPUs start bursting into flames, this is not a problem for Intel.
The problem here is about security. A car analogy would be that to start your car, you need a code and that code can be found by measuring how long it takes to process the input, making life easier for thieves.
As for liability, I don't think you can be liable in court if you didn't plan for something that wasn't known at the time and isn't trivial.