I'm also for increased drug legalization, but it's important to keep in mind the counter-balancing points as well.
Lots of adults will make very poor decisions when presented with the easy opportunity to do so, at least at certain critical times. People fall on bad times routinely (job loss, death in the family, etc) and can quickly find themselves hopeless and/or depressed. In a completely unregulated environment, these bad episodes of life can quickly escalate into serious (e.g. heroin) addiction problems and spiral out of control with devastating long-term effects first on the individual, and then increasingly on everyone around them and the entirety of society.
There's the obvious and immediate costs like ER visits, violence, police calls, etc. There's also the secondary impacts: the devastating impact on friends and family (who may also fall into the same hole as a result), any children of the now-addicted, etc. All of this rolls up into big-picture economic costs and "fabric of society" issues as well.
I have a streak of independence and self-determination in me as well, but it's fallacious to think that there aren't significant tertiary negative effects on others when I do misguided things, and making it super-easy for me to do so at my weakest points in life isn't a winning idea. This is the ethical basis for societal/government regulation of "dangerous" activities and items.
You can see a mirror of these same issues in the gun control debate as well. A bunch of responsible adults reasonably don't want the government telling them they're not allowed to responsibly own firearms and make their own decisions about what firearms they own for what purpose. They can make those decisions for themselves and it doesn't hurt anybody, because they are responsible adults.
But, as above, one has to consider that some small percentage of these responsible adults will eventually have a bad mental health day (or week, or month) and things will spiral out of control. Or they'll make a simple mistake and the gun is stolen by another, or discovered by a curious child or a very upset teenager. These are dangerous items and misuse (or even minor human error in controlling access) can have huge negative societal impacts. Thus, it's ethical for society to take an interest in regulating them.
In both cases (and so many others) the magic is in finding good healthy balance, and realizing of course that law and policy can't prevent all possible harm. Banning all the guns and drugs in the country is obviously not the right answer, and neither is wiping all regulation off the books and letting everyone walk drunkenly into their local corner store and pick up a loaded M-16 and a pound of meth for $20 whenever they feel like it. Developing a good regulation regime that's reasonably-protective and also minimally intrusive on peoples' rights is hard work; much harder than soundbite-happy bans.