That's why DUI is a crime, whether for marijuana, alcohol, or any other intoxicant.
Personally, I'd rather be driving around on a road with 1000 stoned-on-marijuana drivers, than with half a dozen drivers who are drunk.
In any event, such observations have nothing to do with those circumstances when people aren't driving, or are simply enjoying themselves in the privacy of their own home or under some other benign circumstances...
How do they get home from the potshop?