Never heard of an extra charge for towed vehicles, but if State Farm is asking, that may be on their mind. Towed vehicles tend to get stone chips and such - maybe they have been getting too many claims for re-paint jobs on toad cars?
While the car is being towed, it is covered by the towing vehicle's insurance. If it disconnected for any reason, any damage it caused to others would actually be covered by both its own insurance and the tow vehicles, but damage to itself is probably covered only by its own insurance.
2004 American Tradition; 2014 Buick LaCRosse
Homebase in the Ocala Nat'l Forest near Ocala, FL