Billboard advertising is also legal here, whereas I think tobacco advertising restrictions should be extended to include cannabis and chewing tobacco.
You particularly smell it in certain suburbs, especially after-hours and on weekends. However, I think it stinks even worse than cigarettes. To me, you may as well be walking past an open sewer.
What makes me really angry though, is when you smell it in parks. Sometimes next to playgrounds. Mind you, that's just because of individuals. Many suburbs of Tokyo ban outdoor smoking on the streets, and provide designated outdoor smoking areas. Often these are right next to a busy footpath or playground, therefore defeating the process in the first place.
It's worth noting that weed is only legal at the state level. When signing up for a Social Security Number, I had to enter a federal building with airport-level security. I walked past a VERY sorry looking guy in cuffs who didn't realize it was illegal to bring a pipe into a federal building. Priceless. :D