I don't think it is outlawed world wide. I'm sure you could get away with smoking in Antarctica for example.
A better way of putting it would be it's illegal in most modern places in the world and allowed for medical uses in some places.
We can speculate about this all night, but we will never know what really happened unless the owner of the site comes forward. It's obvious that he had many sites dealing with the same subject, maybe something he was doing on one of them lead to overgrow getting taken offline as well.
I should also point out that 'overgrow' in the vBulletin admincp is not there to give credit to that site. It's there because that is the name the author of the karma hack for vB2 used, at the time he coded it he was the technical admin at overgrow.com. I think he left the overgrow.com community about two years ago.