Let us consider the impending end of the War on (some) Drugs, that is, the availability of drugs considered to be pleasurable according to some peoples' ideas. I can think of some objections, and at least one from a very strange angle: junkies "on the Program", that is being "maintained" on methadone or LAAM.
Let's say we allow people to take opiates, according to their wish and penchant. We tax them. At the same time, methadone maintienence programs will most probably suffer, since opiate prices will first, drop through the floor, then skyrocket once tax dollars kick in. (Do we have cigarette maintenance programs for the people who would otherwise prostitute themselves for a smoke?)