True. And a natural (economic) consequence as well. Although 'legalize everything and advertise what works' seems even more politically unfeasible, than 'only unban what's safe'.
It came from an argument (in my head ;) based on the proposition that you would ideally internalize adverse effects in the price of a good. So alcohol-users would pay for alcohol related mischief, LSD for LSD, etc. You would need a product specific VAT for the larger categories. Can't really implement that for N designer drugs, so I thought 'ban the rest for easy of acceptance'. Your effect is more beautiful though.