Exactly
On a serious note, whilst cars stopped in the central lanes of a freeway is obviously a dangerous situation (and no-one here wants to see.) Drivers falling asleep at the wheel is probably a higher cause of accidents.
Sleep-deprived driving - Wikipedia, the free encyclopedia
So Tesla and the AP system are in a good position to help in this regard. As AndyW suggests the car can attempt to pull over, but without rear radar/camera (can they use the backup camera?), it is doing it somewhat blind. Proactively attempting to wake a driver to take control wouldn't be an unreasonable thing to add to the Model S's arsenal.
Probably a loud alarm clock sound rather than the radio might make more sense though
(If for no other reason than to prevent a 100 page thread on TMC about the correct song to be used
)