The Decriminalization Fallacy
Illegal drugs are illegal because they are far more addictive than "legal" drugs such as alcohol or tobacco. That is not to say that they are all equally addictive, some are far more addictive than others obviously. Marijuana is much less addictive than say "crack" cocaine, but it is still more addictive than Vodka. Drugs are also very dangerous for young kids and young adults whose brains are still developing, as they can severely disrupt normal brain development, leading to an increase in paranoia and schizophrenia, as well as ADD and impulsive risk seeking behaviors.
But even if one were to make the libertarian argument that drug use harms only the drug user so it should be up to the individual to make that choice, that is again false. Drug users will drive under the influence and kill people and damage property. they will go to work under the influence and make stupid mistakes that either cost their employers a lot of money or get themselves or their co-workers killed. They will have drug addled unprotected sex and bring unwanted children into the world with no means of supporting them. These children will be neglected and abused mentally, physically, and sexually. they will also pass STD's along to their partners or contract them themselves, further draining the welfare state. They will be unemployable so they will be drains on the welfare systems as will their offspring and sexual partners. So arguing they will only be hurting themselves is a lie.
Others make the argument that legalization and regulation to keep it out of the hands of juveniles the way alcohol is sold will increase tax revenue as well as dismantle the drug cartels and their reign of terror on Mexico and Central America. this sounds good in theory but in practice this will not be the case. Some will not want to pay the tax, others will want the drugs but will be unable to buy them because they are too young, or the quack doctor writing the script won't write it for as much as they want, so the black market won't go away. And the attempts to control the black market won't either because the government won't want the competition undermining their newfound tax base. So the drug war won't go away, if anything it will get bloodier because of the added profit pressures.
So we are left with only one justification for legalization that makes any real sense, those calling for it just want to get stoned but don't want to risk going to prison. THAT is the only motivation that makes any kind of logic at all.