Why is weed illegal?
Weed was made illegal because the government had a lot of the structures and bureaucracy left over from Prohibition and they needed something to do. Meanwhile, Mexicans and blacks were using marijuana which made that substance open to all sorts of racist attacks that were completely acceptable to the population at the time. Plus they were able to pull all sorts of "facts" out of their asses without anybody asking them what credible research was done to prove it, because, you know, DAMN MEXICANS!

Then the US forced its policy on every other country without a good reason to do so since, well, that's the American way. Watch the movie "Grass" if you want to know more about the ridiculous reasons marijuana is illegal.