Well, its always been common knowledge that the Americans started banning drugs for racial reasons. Opium smoking was possibly the first to be targeted - as the opium dens in San Francisco and many other cities were run by the Chinese. The lawmakers at the time naturally couldn't give a monkeys as to whether or not opium smoking was detrimental to ones health - they simply saw it as an unwelcome feature of Chinese culture amongst the expatriot ethnic minority, outlawing opium dens through the use of racist propaganda, stating that the Chinese were primarily motivated by their use of opium dens as a way to lure in 'white' women who they would then rape once intoxicated. Of course, there was absolutely no evidence or basis for these accusations but by creating a moral panic around the frankly innocent Chinese, they could eradicate this highly cultural practice.
Around the same time in the south, the abolition of slavery had done nothing to reduce the entrenched hatred towards the black community. As the use of cannabis and cocaine was popular among the black community - they used the same bullshit that had managed to close the opium dens on the west coast - black men were already being stereotyped as unintellgent second class citizens who posed a direct threat to 'white' women, and the use of cannabis and especially cocaine would do nothing but amplify the crazed, perverted natural tendencies of the community, so again, using the increased risk of sexual assault while under the influence as a scare tactic, they outlawed the use of such drugs as they were 'black' behaviours that had no place in the 'new south' following the civil war.
I may be mistaken in some of the specifics that I have highlighted but it demonstrates the broad strokes of the original motivations for prohibiting the use of opium, cocaine and cannabis - none of these drugs were banned because of the potential health risks that they caused, they were simply a way to try and eradicate and marginalise cultural practices among certain BME communities, so it comes as no surprise that Tricky Dick had similar motivations behind his 'war on drugs'. The Americans have always made sure that powerful psychoactive drugs are available to those that want to use them, just as long as they come from Big Pharma. Where else in the world would you find an average domestic medicine cupboard containing both a powerful benzodiazpine (usually Alprazolam or Clonazepam) AND a powerful Opioid (usually Hydromorphone or Oxycodone) as standard. And its all very well for Obama to start criticising drug policy now he is lame duck as he doesn't have to worry about votes anymore. I personally like the man, as much as one can like a head of state, and to be fair the Americans have become extremely progressive under his presidency at least as far as cannabis is concerned. But he still won't commit to making federal policy on the issue, so despite his refreshing honesty regarding his cocaine and cannabis use as a youngster he still hasn't made any real contribution to drug policy, and for all the progress they have made on a state level, all it needs is a Republican victory this autumn for cannabis to be banned again. In fact, I believe that it should be a priority for the right wing to stop all this legalisation nonsence at a federal level, as we dont want folk smoking cannabis as it encourages free thinking and caring for fellow human beings, which sails far to close to socialism for the good old boys, neocons, the tea party and the like.