I think you're missing a fine point here. You've already said that contaminated meat is criminal, presumably because it's very dangerous. Are bath salts very dangerous? Meth? Low quality meth with contaminants?
How should we permit the sale of narcotics while disallowing poisons? I think that a lot of people can answer that, but how can we do this in a reasonable way? Let's think it through... I don't have time to keep track of which things are safe. I'd probably rely on some organization to help me decide. It's reasonable for me to rely on the government because they're democratically accountable to me. In that system, I'd prefer them to really err on the side of caution. Erring on the side of caution, is heroin poison?
Do you want to live in a world where buying a simple OTC sleep pill at Walgreen's exposes you to the risk of ingesting heroin or some other decidedly narcotic substance?
I really support the legalization of certain drugs, but I'm advocating for a per substance approach. (Actually, I'm saying that it's functionally impossible to just "legalize drugs".)