If you check you will find the break down cover people will only attend to vehicles that are road legal ie taxed insured and MOT'ed this is to cover themselves in the case of a roadside incident and their risk of complicity in the crimes.
The legal state of vehicles can easily be checked by a few click of the mouse these days and probably is automatically undertaken as a part of the process of arranging the assistance
Oops, that was just slightly tongue in cheek