It's true that that the threat of jail often doesn't get people sober or free of addictions. But it does work at times. What's the alternative? Give them free drugs so they stay addicted for life? Of course the rate of addiction is going up faster than those being helped. But that doesn't mean no one is being helped.
It doesn't matter why there are laws against weed on the books. The DOJ is suppose to enforce Federal law. That's their sworn job, and the President's too. If you don't like the laws, get Congress to change them.
You seem to locked into the mindset that people who take drugs are criminals. And I don't mean that just from the point of view that some laws are being contravened, because that can easily be remedied by repealing those laws. I mean that you actually seem to believe it, the way you keep talking about a "stick". Think back, there was a time when people who drank alcohol in the US were declared to be criminals, something which also didn't work very well. I hope you'll agree that alcohol prohibition was basically a nutty idea.
There is no need for a "stick" in this scenario, because viewing the issue from the criminal point of view makes no sense. People will do things that are not good for them, and humans have always devised ways to help mitigate that. Drugs are no different and it serves no purpose whatsoever to compound the problem by criminal action.
Your self-stated moral objection to the government providing drug relief (such as safe injection sites or other related programs) is nonsensical and is based on the notion that using drugs is, in itself, a criminal act. But that notion is an arbitrary one, not based on reality. Why would you morally object to handling a problem in a cheaper way that works better? It seems to me that you should have a moral objection to spending money unnecessarily through criminal action when it itself obviously causes so much harm, and has not only NOT helped solve the problem but has made it worse by increasing the cost of the crime, and thus increased the power of the criminal gangs who control drug distribution.
People who self-administer drugs often harm themselves, so doesn't it make sense to direct help to them specifically? By arbitrarily making them criminals, and making them dependent on criminal systems for drug delivery, that ends up causing harm to the rest of society, something that is utterly unnecessary. It simply makes everything worse for everyone.