He also laments the fact that nothing has changed since he wrote his last article a year ago.
Yet perhaps he is missing the point. The persistence of criminal statutes against the production, distribution and consumption of recreational drugs may have less to do with purported health concerns than with the usefulness of these statutes in maintaining and expanding certain political powers and furthering certain people's careers and financial interests. So even if the DPP win the next set of elections, and even if they agree with his views about the relative harmlessness of cannabis consumption, there is ample reason to believe that nothing will change.
The drugs themselves and the consequences of their consumption are less important than the political power and resultant benefits their criminalization affords those in (and around) public office.
Further to that point, I would argue that it makes more sense to argue for the decriminalization of cannabis and other recreational drugs rather than their legalization. Legalization would merely plant new seeds out of which political power can grow again through the taxation and probably ever more tangled regulation of the products, whereas decriminalization would not. Decriminalization of cannabis and other recreational drugs would help to weed out the growth of unnecessary political powers.
On the author's appeal to science, I think this is misplaced: as Hume taught nearly three hundred years ago, the "ought" does not follow from the "is". And true scientists are exclusively concerned with questions regarding what "is", and not - in their capacity as scientists - with what actions "ought" to be taken. Although a moral decision may be at least partially informed by factual evidence, factual evidence alone is insufficient since, as Hume pointed out, a moral decision must ultimately follow from distinctly moral premises.
The moral premise in this case is that adults are intelligent agents exclusively capable of acting in their own best interests, since those interests are theirs to judge in the first place. It follows from this that people must be held responsible for the consequences of their own decisions and not be treated like permanent children or the mentally deficient by political masters who care not for the need to limit political powers.