The problem with drug laws is the problem with all laws - violent repression of activity. Sometime violence is called for, but it can never be called for simply to control a stranger's non-violent, victimless behavior because you think it's in their "best interest".
Even if that behavior is negative, and leads to broader problems in society, we have to ask the question - is the initiation of violence against non-violent people the best way to solve the problem?
What are the predictable consequences of using violence against people for doing a drug? Any drug? Let's take something that no rational person can support - heroine.
Laws against heroine directly do three things:
1) Causes those who use it to fear their government.
2) Costs a lot of money to enforce the law, which could be used for other purposes.
3) Creates a hidden, violent, and dangerous black market, and being hidden means being unaccountable.
The motivation for keeping these laws is the same motivation for enacting them -- to reduce the drug from society, and to reduce the crime associated with drug use and abuse.
While the general population accepts these reasons, we still have drugs in society. Is this number reduced if we spend more? Well, in the decade of 1990-2000, there was actually a positive correlation.
Cost of the war on drugs, for ten years - 1990-2000:
Drug use by high school seniors, 1980-2000:
More money spent. More drugs used. Funding the war on drugs just creates a spiral of justification for more money and more power.
Besides, what about all of the violence created BY drug laws? It was prohibition that created a murderous thug like Al Capone, and laws against any other drug create hundreds of these people.