Because if there are forces being applied to the tractor other than just a dead pull, when the tractor goes over lumps or crosses ruts it can lead to a jump in the mole which is not good.Why should the moler not pull down on tractor it's not as if the tractor is lifting it its just the forward pull on the arms that pulls the tractor wheels onto the ground. Maidwell moler mounted takes some beating can back into ditches and lift up and let it ride out at the ends
Same reason why you must never stop with the mole in the ground unless you are just downhill of a drain