But, in any case, it is your obligation to leave as much space as it takes for you to stop your car before it hits the car in front, taking in consideration the speed of both vehicles before braking and the condition of the road. The rule of thumb is there for you as a baseline; blindly following it does not automatically mean that you're in the right if an accident results.
[1]: https://assets.highwaysengland.co.uk/stay+safe+stay+back/HE_...
Pedantic analysis of DMV regulations is absolutely essential. Clearly any dolt practicing defensive driving would anticipate the self driving car cutting him off and hard braking.
Stories like this make people think that full self driving is actually a car the drives itself fully. The car is safer than any human and by critically thinking about this we are preventing sales and essentially killing people, since Tesla has fewer deaths per mile travelled.
It's also not just some arbitrary rule, it's a fundamental principle based on the rules of physics.
