Presumably they were challenged to prove their whereabouts at the time of the crime.
Interestingly, though, the registered keeper of the car might (legally) have the option of taking the rap - as I understand it you can refuse to provide them with the information as to who was driving the car at the time, and this carries a penalty (possibly more severe than the speeding penalty), but provided you don't care about having a record and are a non-driver, who cares? Was thinking my Dad might ideal

Then I remembered that in my 24 years of driving I have never even been stopped for speeding, let alone been caught, so I guess I'll wait until I have at least 6 points before worrying about it
Cheers,
Robin