You seem to be arguing that legalization should not be determined by a criteria of "acceptable risk". If risk is to remain a factor, the it will have to be as an absolute - "risk" or "no risk".
For example, the arguments have gone, a) Marijuana is not as harmful as alcohol, b)Alcohol is legal, therefore c)Marijuana should be legal because it's not as harmful. I am not swayed by this argument, and it's an insult to my intelligence when it's made. I think there needs to be better arguments than that before people will vote to overturn the current laws.
What do you consider the appropriate criteria?
If marijuana were legalized, then we can look at where it fits in with the other legal drugs (such as alcohol) in order to determine how it is to be distributed.
If legalization is to be determined by risk, in absolute terms, then prohibition should be maintained if there is any risk. The decision to legalize would have to be based upon a determination of an absence of risk, and you are arguing that we should not make any determinations about acceptable risk until we have first determined that there is no risk.
Maybe this will help (and I've tried to restrict this to recreational drugs to simplify things). Legalization of marijuana should not be determined solely by a comparison to the risks of alcohol. In other words, just because marijuana may be "less risky" than alcohol is no reason to legalize it.
Furthermore, "risk" should play no part on whether or not a drug is legal. (Many legal prescription drugs are very "risky", and their usage has to be monitored closely).
So, in summary, risk is not a factor in determining the legality of a drug.
Now, once a drug has been declared legal, we can then consider risk. Very risky? Prescription or research only. Little or no risk? Over the counter. Anything in between can be handled by age restriction, quantity availability, licensing of distribution, etc.
"What do you consider the appropriate criteria?"
I have no desire to see marijuana legal. I used to feel that the arguments for medical marijuana and decriminalization had some merits. But the more I'm exposed to it, the more I see that these are just ploys towards eventual legalization. Nevada is a good case in point.
I have yet to see one good argument on why we should legalize (just) marijuana.