I don't see why marijuana couldn't be legalized and controlled in much the same way as alcohol.
The difference lies in the residual effect of both and the risk factors for the population as a whole. Somebody stoned on marijuana is certainly no more dangerous to himself or the population as a whole than is somebody drunk on alcohol.
However, alcohol, even in high quanities, clears the system within 24 hours and any residual hangover lasts no more than another 24 hours. Ergo, an airline pilot can be drunk as a skunk on Saturday night, and be perfectly sober and cleared for duty by Tuesday morning. And there will be no blood alcohol detected.
Marijuana, however, remains detectable in the body for up to 30 days and there is no known test to determine when a person last used it or how impaired he or she might be. And that makes it far more difficult to regulate than alcohol. For that reason, if we legalize the stuff, and I am not opposed to that, I would want the authorities to back employers and regulatory processes to the hilt to have zero tolerance for marijuana use in all professions where a clear head and quick response time is important.