If you look at the country regions, you would need to control for multiple different factors within that society, so the study done in New Zealand is the exact study you're talking about. That is where they were able to control for all of the different things, and basically had identical people, half of whom used, half who didn't, and compared those groups when controlling for other things. I think that New Zealand study is really the study you're referring to.
If you were simply to look at a part of the world that has higher cannabis use, let's say Canadian youth versus youth in India or youth in China, you would have to be controlling for so many things it would be almost an impossible research design. You need a much more natural group to look at, where you can control and fix things, and that would have been the New Zealand study.