TL;DR potheads don't know how to do shit.
Now take in consideration that, unlike other stuff you could do while being bored (eating like a pig or jacking off), smoking weed is seen as something that le kewl kids do, and it's much more addicting. Plus, you never get any immediate side-effects from smoking too much in one day, and this pushes the majority of people to do even more pot. This doesn't happen with eating like a pig because you feel very sick the next day, or your balls hurt from jacking off too much, or even having a huge hangover from alcohol stops the majority of people from becoming addicted.
Since we are in a gaming forum, let me put it this way: imagine you are playing a RTS, and you need to search for technologies in your main building while you don't make more units. But, instead of this, you just research "420 blaze it" that does absolutely nothing other than make you feel good. In the late game, you are thousands of years behind everyone because you got high all the time rather than doing something useful. And, since pot doesn't have any immediate side-effects (it's rare to get a bad trip anyway), you have no penalties and nothing stops you from doing it over and over again.
Potheads might argue that they are fine and that they don't do it all the time and that I'm a jesus-lover muslism bigot virgin no-global pro-choice or whatever insult to try validating their argument, but tell me: how many people you guys know that smoke weed and, on average, how many of them are successful?
I have nothing against getting sick people high all the time. But the majority of people who smoke don't have cancer, so please let's not talk bullshit. The majority of smokers are TEENS and they do it because they are BORED, and they should stay away from this shit while they are developing themselves. You can't grow up with weed.
Oh, this is EoF? Uhm... the jews' fault I guess?
EDIT: and by the way, drugs in Italy are imported by the mafia. So in a way yes, you do support terrorism buy purchasing it. If anything, you should grow it yourself.