Australian drug policy is almost never strictly an australian issue. Any attempts to liberalise drug policy in australia has always met with interference from the USA. Even scientific trials of supervised heroin injecting rooms were significant enough for the USA to send people out here to put pressure on our politicians. One state in australia produces a large percentage of legal opiates via poppy farming and basically anytime any moves are made to look at alternative drug policies we are held to ransom by the USA who can veto our licensed opiate production. The USA even sent their drug czar over when there was talk of a heroin [instead of methadone] supply trial.

So while I don't see the USA having any direct interest in these laws, I feel that at the very least their constant meddling and influence has removed any politicians from office who are likely to challenge the current policy. ie if you remove the effective opposition to the general policy then you will still get the desired outcome even without the need to get involved in the finer details of that policy over time.