It's because when the engine is stationary, neither the input shaft (red) nor the countershaft (blue) are moving. Only the output shaft (green) would be moving. The reason this is a problem is because the transmission is either splash lubricated via a submerged countershaft or lubricated by a lubricant pump that is usually driven off of the countershaft. If you go too long with the vehicle coasting out of gear with the engine off (as are all towing situations regarding wheels on the ground) the output shaft will be spinning and spinning without receiving any lubrication from a spinning countershaft.Hummmm, I know my Miata... I have to flat bed tow it, I've heard that if you tow a RWD car it will destroy the transmission. I guess because it spins the drive shaft but no coolant nor transmission oil is flowing. I'm interested in the answer to this... I wonder how the owners manual says to tow it.
That's also the reason why the manual tells you to put it in gear. When you keep the trans in neutral the output shaft will start to rotate freely just from the friction within the transfer case. Then you'll be wondering why you can't drive your Jeep with four flat-spotted tires.