I could be wrong, so if someone knows better feel free to correct me.
I'm quite certain if you tow a vehicle with an auto transmission for an extended period of time, and at highway speeds, you will damage the transmission. this is due to the fact that the mechanism that ordinarily drives the fluid pump, which is coupled to the flywheel of the engine, would be stationary, ie, not pumping the trans fluid. that means whatever is spinning around in the tranny likely isn't getting sufficiently lubricated.

older cars (50's-60's models) had pumps tied to the output shaft as well as the input shaft which is why you could push start them, but such is no longer the case. if you plan to tow a car with an AT, it would be wise to keep speeds below 35 mph and the trips short, or remove the drive shaft(s).