Mark Kleiman elaborates on his reasoning for why marijuana should be illegal to sell, but not illegal to possess--he doesn't want to see a fully commercialized marijuana industry:
So the prospect of a legal cannabis industry working hard to produce as many chronic stoners as possible, and fighting hard against any sort of effective regulation, fills me with fear. I don't believe that the actual tobacco companies would enter the cannabis market, but I don't doubt that the cannabis companies that would emerge from full commercial legalization would have all of morals the tobacco outfits morals, and a less tainted product to sell.
E.D. Kain doesn't like the idea, and prefers outright legalization and commercialization:
Furthermore, I’m much more afraid of violent drug dealers, over-eager SWAT teams, and the whole awful black market cycle of violence than I am about the lobbying arms of a few big corporations which apparently fill Kleiman with fear. I’ll take lobbyists over drug cartels any day.
I think Kain is missing at least part of Kleiman's point. The whole idea behind decriminalizing marijuana possession is to eliminate the "black market cycle of violence"; since people wouldn't necessarily be dependent on dealers, dealers would have a hard time plying a lucrative trade, and paramilitary SWAT teams wouldn't be shooting dogs and old ladies trying to get at the hidden cannabis stash of a 72 year-old with cataracts.
Second, while I'm not quite sure where I stand on the choice between legalization and decriminalization, I do think that marijuana abuse is a relatively minor problem. I'd like to preserve that status quo while eliminating the draconian penalties and absurd amount of law-enforcement resources devoted to preventing people from toking. But I think Kain is being a bit to dismissive in arguing that there would be no adverse consequences from the mass marketing of marijuana. It seems entirely possible to me that commercializing the drug could create a problem where none really exists -- businesses have to make a profit; someone growing their own doesn't. A world where a smaller, less profitable illicit market that continues to exist looks a lot like our own without the outsize penalties and adverse consequences of over-enforcement. I'm not sure what a world with a fully commercialized marijuana industry that profits from turning people into potheads looks like, but it makes me nervous.