A lot of people currently partake in either the medical or recreational use of marijuana. Often times these odors and second hand smoke drift into the apartments and houses of people who do not smoke. These can be children or elderly adults who should not be around the second hand material.
A mandatory park for every 500,000 people which is dedicated to the use of marijuana would help stop some of these issues. It would also provide a bit of therapy by being in the outdoors.