No matter how arbitrary and unnecessary? What does anyone have to gain by reporting people for partaking in illegal activities where they are only harming themselves?
You really do seem to think that legal is a synonym for right and moral, whereas illegal is a synonym for illegal and wrong.
If being gay was still illegal, would you report people for being gay if you witnessed it?
To be fair though it doesn't just affect them. You may think its just a bit of weed but the effects are bigger than just the person that smokes it. Unless the person smoking it lives on his own interacts with noboday miles away from anyone else and grows his own