It is great folly to legalise marijuana except for closely controlled medical use, similar to morphine, say.
The large numbers of other countries that may or may not have legalised it are irrelevant.
The numbers of users who may get put in the lockup due to their insistence on using it, are irrelevant.
What is relevant is the fact that it is mind-altering and leaves users feeling they know some secret that others don't know: a situation we call "paranoia".
Users in this state are prone to suddenly attacking others, as we find happens.