There are many things that work fine as long as conditions are favorable, and 4-down vehicle towing is a good example. The risk to the vehicle is excessive heat and little or no lubrication of the moving parts. If not towed too far, or too many hours, or in too hot conditions, or at too high a speed or ...[you get the idea, right?], there probably won't be any disastrous results. Maybe some extra wear & tear, but not enough to become an issue. But the engineers have no idea what you will do when towing your car and have to assume that at least a few owners will exceed what the vehicle can survive.