Why is it that a rear-wheel-drive car cannot be towed like a front-wheel-drive car? What happens if this rule is broken? What about all-wheel-drive cars with no mechanical link between the two axles?
You cannot tow a car with all of its wheels on the ground. To do that, you would need a driver in the towed car to control the steering and brakes. But if the engine is dead, this will not be possible.
As a rule, two-wheel-drive vehicles should be towed with the driven wheels lifted off the ground.
So, a rear-wheel-drive car should be towed with the rear wheels off the ground and a front-wheel-drive car with the front wheels off the ground.
The reason for this is to ensure the driven wheels do not transmit power to the transmission. For most automatic or dual-clutch transmissions, this could cause serious damage because the engine-driven pump for lubrication and cooling is not in operation. This will lead to overheating of the transmission fluid and high component wear.
These problems do not exist with manual transmissions, but it is still preferable to tow with the free wheels rather than the driven wheels to reduce the mechanical drag of the differential and gearbox that the tow truck has to overcome.
Towing a four-wheel-drive vehicle is somewhat more complicated. With some 4x4 passenger vehicles, one axle can be disengaged from the transmission to allow the other axle to be lifted for towing. But this is not a common feature.
Unless the vehicle is a hybrid with a separate electric drive for the secondary axle, there is always a mechanical connection between the front and rear wheels in a 4x4.
In either case, a four-wheel-drive should be winched on to a trailer or flat-bed truck for recovery transport, never by a tow truck that lifts just one axle.