The government may not own our bodies, but we can regulate commerce. This is why we typically go after the supplier instead of the demander. Its nicer consitutionally, however less practical, as when there is demand, there will ALWAYS be supply, and the criminal enterprise that comes along with said demand/supply if the product is deemed illegal. Fighting said supply requires massive use of police forces, which to me is a greater threat to our freedoms than a more limited police force getting a judge issued warrant to determine if an illegal substance is in your system. If you punish the demand enough to lower it, you remove the need for a supply.
It's all fruit of the poisoned tree. When government tries to regulate what we can do with our own bodies, no one respects it. Then as you say there will be supply to get the money. The KKK was done in mostly by racist white males who still thought lynching and attacking black people was wrong, so they cooperated with the FBI. Almost no one turns in someone who just decides to put drugs in their own bodies.
Furthermore, since on this issue the law is not worthy of respect, we are training people to not respect the laws in other ways. I don't do drugs, but it has nothing to do with that they are illegal, and yet I have no respect for the law over this. The law should be worthy of respect.
I agree with you, however excessive drug use does have a negative impact on society, and society does have a right to regulate it, espeically if the abuser has an impact on society. Today drug laws do foster a disrespect for the law, same as laws against 16+ oz sodas, salt shakers, and other nannyist ideals.
There has to be some way however, to remove people from society who cannot "play nice" with drugs.
My proposal would be to legalize them all, however if you get caught with a non-violent crime while on a drug, you get mandatory treatment, and one more chance to use said drug. Second violation, more treatment, and you are now banned from ever using it again. if you get caught using it, its off to a nice camp where u can use all you want, but you can't get out. (Hotel California if you will) unless you go to another camp and stay clean in it for 5 years.
Drug use becomes a privilidge. Mess it up, and you lose said privilidge.