That doesn't make sense. There is a definite/noticeable slump below the norm for all the pot smokers I know (off hand count; perhaps 20-25) across a wide variety of jobs/ages.
Certainly some high functioning people I know could be abusers behind closed doors; but they exhibit none of the standard symptoms so it's hard to imagine that is true... (I will try and do some surveying though :))
In other words; you really need to find a high functioning heavy pot smoker to give the theory credence - and I don't think that is going to be easy.
Well, then we should avoid encouraging fast food, liquor, and a lot of other things
We are already doing that; indeed for several years now there have actually been campaigns to discourage the overuse of those things.