Portugal decriminalized all drugs in 2000 and saw a huge reduction in harm associated with drug use, and did not see an increase in usage. They went from among the worst in the EU in terms of HIV infections to among the lowest. [1]
[1] https://www.theguardian.com/news/2017/dec/05/portugals-radic...
Edit: cannot edit any longer. While I do not think they do cause psychological harm in the long term (I think they force confrontation with buried psychological issues in order to transcend them), they most certainly can cause short term psychological distress and issues for people unprepared, approaching with the wrong mindset, or using in an otherwise problematic set or setting.
That said, psilocybin is highly sensitive to set and setting, especially to other neural nets and especially to others also on it. There is a massive host of issues in partaking with friends or around anyone else for that matter. It will amplify hidden untruths or other sort of otherwise “benign” sober psychological distress and make them acute.
What people call a “bad trip” I call an important lesson. We can choose to react negatively and label it accordingly, or visit it in a sense that a very advanced mindset was indicating a problem. When done this way, bad states cease to exist and it becomes clear the “bad trips” were just bad other things that needed some conscious attention.
All this to say: go get lost in the woods and trip with some plants & with people who hang with that setting, they will take care of you and it is a much better setting to do the work on self.
Except it opens doors in people's minds which allows them to think differently for themselves. This results in them behaving less like sheeple. Why would that ever be scary for society?
Sure, not all mj users may experience it this way, but a lot do! I certainly wouldn't be arguing it opens the door to a bunch of usefully profound thinkers in society.
MJ used more sparingly, and especially when used on someone with exposure to a psychedelic (not simultaneously but maybe the weekend after a trip)? Entirely different ballgame. It can awaken an internal symphony of inspiration and thinking.
Everything can be abused. Eat too much if anything and you will suffer. But when used appropriately, very magical experiences can be had that enable routes of thinking and problem solving that seem to come from out of this world.
In general, my recommendation to anyone is to try these things while simply meditating or hiking a safe & easy trail. Your mileage will vary but when done correctly, you will acquire otherworldly inspiration applicable towards your skilled professions and hobbies.
And if nothing else, even falling short of that result, this possibility should be enough for legality.
An acute example of this is a psychedelic called “DMT”. This psychedelic is nearly everywhere in nature, including our own brains. When inhaled, it lasts less than 10 minutes, give or take. After that 10 minutes, a user is completely sober without any indication that they were not, and no physiological side effects. But during that 10 minutes? It is an acute spiritual and existential experience, possibly the most acute non-normal experience possible for a human being. A true exploration of altered states, and possible contact with a different form of life unbound from our reality.
And for some reason, this substance is scheduled. It does no harm to a user, to third parties, nor to society, is short lived, and offers true spiritual experience but it’s scheduled. The scheduling as a whole is archaic, oppressive, and if nothing else, misguided.
This is also in top of the fact that it discourages addicts from seeking real treatment due to fear of being criminalized.