I think the study misses the mark a little. I do agree that there is some issue around the pleasure factor, and an ingrained societal belief system that admonishes euphoria as somehow hondistic or unbecoming; but the greater reason that the least harmful drugs are illegal is that they cause the user to re-examine the human structure of living. Alcohol and tobacco are the most harmful, but they do not pose any threat to the established and accepted thought patterns.
If you look at which drugs pose the greatest threat to the status quo and which drugs don't, the scheduling of substances will make a lot more sense. It's not about pleasure vs. dependence but acceptable thoughtspeak vs. unacceptable ideology. People whose point of view is shifted dramatically may refuse to participate in, or even begin rebelling against the structures they grew up unconsciously accepting.
If the laws were genuinely about harm reduction then alcohol and tobacco would be schedule I. That's not to say that there is no harm to the other substances, but in terms of loss of life, and socioeconomic impacts, alcohol and tobacco take the cake.