1) It is hard to tell if someone is under the
influence of cannabis. Driving/working with
machinery may be problems here.
2) People might smoke too much and become a
little lazy.
3) Many governments have prohibited it for over
70 years and it would seem very stupid if they
told everyone that they where wrong about it.
Now let us look at these points for a moment.
1) Yes it is hard to tell and make no mistake -
if you drive smoking cannabis then you are just
as stupid as if you where driving under the
influence of alcohol. It is never advised to
use cannabis and then perform a procedure, like
driving, which requires your total attention.
As of yet there is no on the spot breathalyser
test for cannabis use, but there are tests to
determine if someone has used cannabis
recently, in the last few hours, in the last
few days, weeks or months. However, alcohol is
not banned in a lot of countries but cannabis
is.