The title of the uop is misleading.
1. Drug cartels have been growing pot in California and other states well before it became legal in California. When pot became legal there the cartels just continued to ship weed from several north western states and northern California. The red states supply most of the illicit tax free revenue.
2. By legalizing pot, California has established some control in ensuring the growing process of the legal plants is safer for users and the environment. Contrary to what the op implies, it is the refusal of the red states to legalize marihuana that drives the negative environmental and health issues caused by illegal growers using powerful deadly toxicants to kill animals that eat their crops.
3. Cartel Grower houses would likely produce a safer product without pesticides as opposed to the illicit plants cartels grow in our national parks and forests where pesticides reek havoc on the wildlife and ecosystems; Not to mention the deleterious effects passed on to users when they smoke MJ or bake cookies with the pesticide laced produce.