In the USA, many rental car agencies have clauses prohibiting driving on unpaved roads: for example, see those questions on Travel.SE: (1), (2), (3). From one example agreement including some causes many renters are likely to be in violation of:
Prohibited vehicle uses and activities include:
Using a vehicle (...)
other than on paved roads (whether "off-roading", driving on unimproved roads or parking areas, or otherwise)
to transport any flammable (...) substances
The first point implies that any car (even a jeep) cannot be taken onto unpaved driveway or parking lot. The second point implies one can't have fuel in the fuel tank, and even if that's likely not what they mean, it means one can't bring a lighter or camping fuel. Many are likely to violate both items (for example, many tourists in rental cars park at the gravel parking lot for the popular Antelope Canyon tourist attraction, knowingly or unknowingly breaching their rental contract).
A comment to a now-deleted post stated that If you use a rental car in an unauthorized manner, your right to use it terminates, and you are driving in a stolen vehicle.
Is that right? If someone has signed a rental contract stating prohibiting the use of the vehicle other than on paved roads, and then violates this by driving the car onto a gravel driveway or parking area, does that mean the renter is guilty of car theft?
For the purposes of this question, you may assume California, USA, but other answers are also welcome.