the first one doesn't even go into marijuana use, it alludes to a "commonly know fact" but gives no reaseach findings for this and then delves into how the brain functions.
The second one describes correlation between use/age and brain matter but as we all know, correlation DOES NOT MEAN CAUSATION:
"Decrease in hippocampal volume in regular cannabis smokers has been correlated with lifetime consumption (Ashtari et al, 2011; Yücel et al, 2008) and psychotic symptoms (Yücel et al, 2008). In Cousijn et al (2012), volume reduction in the amygdala and the hippocampus does not differ significantly between regular cannabis users and controls but still correlates with the amount of cannabis used and the severity of cannabis dependence, respectively."
the third one is is about adolescence use and how it can impair brain development, which is pretty much true for anything given (like alcohol) or not given (proper nutrition) to a developing brain.
So i guess not smoking makes you stupid too, at least that's the correlation i find when reading your post and using your standard for judging the same.
" It also lowers reflex response time, and it makes you apathetic."
Not getting enough sleep does the same thing, have you ever stayed up late? because staying up late is STUPID.
see i can make egregious arguments without any rationality too.. fun right?