But what vulnerable people are affected by the legalization of various items, like marijuana or arsenic? That's what I'm not following. You can criminalize putting arsenic in food for consumption without proper labeling, for example, and ensure that it's not legal to harm people with arsenic. Truthfully, with respect to arsenic specifically, it's an extremely moot point, because no one would buy arsenic for consumption...
As for marijuana, again, what vulnerable people are hurt by it? If it's addiction, again I gotta point to other legal things like alcohol. This link from alternet discusses the question of marijuana addiction, and cites a report supported by the National Institute on Drug Abuse claiming that only some 9% of marijuana users become addicts, compared to 15% for alcohol and a whopping 32% for tobacco, another legal drug. ( http://www.alternet.org/drugs/151991/is_marijuana_addictive/ )
I'm still not following the cultural argument. Culture should not affect the legality of drugs. If you're talking about culture to explain why it's the case that some drugs are legal and some aren't, okay, I buy that, but it doesn't affect whether it *should* be the case.