Partly correct. The act of smoking it in a controlled environment and it's effect
may not cause harm to others. However, it's use scientifically through chemicals introduced voluntarily into the bloodstream causes physical effects which can and do impede the judgement of an individual - the corollary is alcohol as the physical effects can and are sometimes impeded depending on the
amount of THC or alcohol introduced over a period of time. That impediment to judgement provides opportunity to harm to others. The issue is not the action of driving while under the influence but that one is under the influence of a hallucinogenic drug. DUI with alcohol is illegal as is DUI as a result of smoking marijuana.
The crux of the harm is not driving but the use of a substance WHILE driving - back to "impeded judgement". Therefore the "potential" for harm is greater driving under the influence of any drug than not.
The use of legal narcotics I would add, such as prescription drugs can have an equal or greater effect on an operator of a vehicle whereas prescription drugs are provided to users under a controlled environment with consultation of a doctor, sold by a certified pharmacologist with FDA warnings and instructions on it's use. THC is scientifically, a narcotic and therefore should at least be controlled with as much care as an existing prescription drug.