No one's saying we shouldn't have laws.
I think we can all pretty much agree that we're all guaranteed to life, liberty and the pursuit of happiness and all that jazz. This DOESN'T mean we're allowed to do whatever the f!ck we want, but it DOES mean that we're allowed to do whatever we want provided it doesn't harm anyone else.
So, how exactly is it acceptable to legislate what substances someone puts into his/her own body? It's not. We don't tell people they can't eat fatty foods, smoke, or drink. It would be absolutely unacceptable; if adults choose to engage in these activities, they are assuming the personal risk that comes along with them. We can create laws that prevent them from harming others while doing so...secondhand smoke isn't okay, and neither is drunk driving. Doing drugs? I don't believe that's any of the government's business.
Anti-drug laws are ridiculous and I would never vote for a politician who supports them. If my colleagues, friends or neighbors choose to smoke weed, it's none of my business unless they're harming me or others. It's usually pretty easy to get people to agree to the "live and let live" argument, but for some reason, when it comes to drugs, people flip their ****. As if legalizing cocaine is suddenly going to make it more attractive to people. I don't not do cocaine because it's illegal...I don't do it because it could kill me, duh. If the dude the next door wants to take that risk, why is it any of my business? It doesn't affect me. If he gets violent when he's high and tries to cook his children, then sure, that's a problem, but our #1 priority when drafting legislation should be guaranteeing actual liberty, not preventing potential harm.
And even if you don't buy the ideological/political arguments, the economic ones are solid. We waste billions of dollars a year fighting drugs when people ARE GOING TO DO THEM ANYWAY. There will always be a demand for drugs, so we might as well take advantage of it and let them be bought and sold on the market just like anything else.