The drugs (policy) don't work

So it seems likely that the government that brought you 24-hour drinking and gambling is going to do a wee U-turn and regrade cannabis back up to a class B controlled substance. The 'debate' about drugs is usually pretty dismal because practically no-one deviates from the (unfounded) assumption that the present legislation grades narcotics according to the harm that they do the consumer. So-called 'libertarians' usually don't make a proper liberal argument at all; instead they accept the logic but attempt to counter it with some hippy nonsense about cannabis being harmless, or relatively harmless.

Leaving aside the debate between real libertarians and legal paternalists, why does everyone assume that down-grading, decriminalisation or legalisation automatically increases demand? Is there really a vast army of people out there who aren't banging heroin into their veins only because they don't know where to get it or are fearful of the legal consequences? And even if the prohibitionist argument were to be accepted, surely it isn't being executed very logically? Here I had to agree with Simon Jenkins on the wisdom of the British government expending quite so much military manpower on the task of blowing up poppy fields in Afghanistan:

"Trying to combat Britain's addiction to heroin by burning poppies and smashing opium "factories" is like combating London's traffic congestion by bombing oil wells."

There are, for me anyway, two escapable facts about drugs that any policy-maker should get real about: demand is not going to go away, period - but drugs are harmful so this demand has to be regulated, in proportion to the harm this does on the wider society. Maybe if they understood the former a bit better, they'd think of more imaginative ways of doing the latter?