Walmart and Amazon are hyper-efficient organisations that are able to deliver extremely low-cost goods. Hence their popularity. If ultra low prices are what consumers want, then what's to stop someone attempting to sell marijuana that cheaply? Again, not really a vicious cartel's forte. A cartel's forte is violently excluding competitors to maintain high profit margins in a market hidden from normal legislative balances.

As long as all the growing takes place within the legalisation territory (i.e. nothing is sourced from illicit operations in foreign nations) then I see no reason why violent organisations would dominate legal trade of psychoactive substances.