The effects of THC aren't an on/off kind of thing. There is a huge range of middle ground between baseline and high in the sky. You wouldn't call someone medicated on Adderall as prescribed by a doctor unfit for driving, either, would you? Yet a person self-medicating the same dosage without prescription is suddenly a danger to others. Which is insane. Same thing goes for THC IMO.

Not to mention that THC's psychoactive effects can be completely blocked by sufficient amounts of CBD and probably other compounds. You can try it yourself. Get CBD drops, ingest 100mg and try getting high. Even the most potent sativa will not work as you might expect.