I agree, and this is one of the main reasons I'd prefer to see decriminalization rather than legalization. Once the cigarette companies get their hooks into marijuana, I can't see any reason why they wouldn't start loading it up with flavorings and chemicals and preservatives and going for a selling price that's ten times the cost of goods. I expect they'll have to budget for packaging costs, legal costs, marketing costs, distribution costs, and - wait for it - lobbying costs, just like they do now with tobacco. But it will be the slow and steady deterioration of the product that will be the real buzzkill.
Also, it wouldn't surprise me to see Monsanto come along and patent a hundred varieties and sue the tie-dyes off of anyone who ends up with one of the genes in their backyard grow.