Well, think about this. THC stays in the bloodstream for up to 30 days. You have a commercial pilot, driver, etc., that has used marijuana and has some level of THC in their bloodstream. They get into an accident. Let's say it's an airliner and 100 people are killed or very seriously injured. That's a accident lawyer's dream case. Pilot was higher than a kite (doesn't matter what the THC level was that's what the lawyers suing will claim) and caused the accident making the carrier libel for millions.
Unlike alcohol where it is pretty quickly passed out of one's system, THC from marijuana use isn't. So, the pilot smokes a joint on Friday and his next flight is on Monday. Sure, he's 99%+ sober and the THC isn't an issue, but the plane crashes and they find it in his blood stream (dead or alive). That airline is done, finished. They're forking over millions and any rational person knows it.