An electric car still has a transmission, which is a gear system in a container of oil. That system produces heat and must therefore be actively cooled, this is done with a heat exchanger. If the main system is defective due to the loss of the 12V, then the heat exchanger will not work either and the 'transmission' can overheat, resulting in damage. If all goes well, only oil will be burning, but if it goes bad, you'll have leaking gaskets and other heat problems.
A second aspect is that a battery in an EV is not as simple as 3 AA batteries in a toy car. To protect against fire and degradation, the battery is actively controlled by the so-called BMS. That also works on the 12V. No working BMS means no 'access' for power from the motor to the battery. So if one generates power by towing the car, it will have nowhere to go and you will get voltage peaks. Ergo, the car will prevent this via a blockade.
(Source: Waarom de 12V-accu belangrijk blijft in een elektrische auto (autoweek.nl)
