On many automatic transmissions, the fluid pump is driven by the engine, and putting the car in neutral just disengages the transmission from the engine. When the wheels turn, the transmission's internals (for example, the output shaft) are also turning--but without the benefit of lubrication. The effect is similar to what would happen if you were driving with no transmission fluid--within a few minutes the fluid around some of the bearings would get hot and boil away, and it would not be replaced. Then the bearings will heat up, wear away, and eventually seize. By the time you get up to highway speed you're already doing damage. That is, if you have that transmission design.
Best bet, if you have to tow an Automatic on a tow dolly, is to disconnect the drive shaft. Second best bet is to put the drive wheels onto the dolly. If that means putting the car on backwards, just make sure the steering wheel is straight and secure. Otherwise it'll dogleg as you tow it.