Illicit drug use

Of course, I can't be sure in the case I mentioned that drug use really caused the guy to hold up a convenience store. The correlation between drug use and violent crime may not reflect any causal relationship. There may be some hidden variable, like a lack of character, that causes people like him both to use illegal drugs and to rob stores at gunpoint.

Absolutely agree with that. A person can't say, "Oh, I pulled that robbery because I needed drugs" any more than a person could use as a defence, "Oh, I was drunk when I hit that pedestrian, so it's not my fault." We are held responsible for our actions regardless of what chemicals are in our systems.

How about "I was too drunk to know I shouldn't drive while drunk".
A clear case of temporary incapacity.

Or,
"I was too drunk to know I shouldn't vote while drunk"
A clear case of Obama voter remorse.

I certainly wouldn't contest that (although the convict I mentioned would).

My point is there's still a causal relationship between putting mind-altering chemicals into one's body and the crimes one goes on to commit. So there's a social cost in allowing unlimited drug use: people who are strung out go off and commit crimes.

You say, "people who are strung out go off and commit crimes." Sure. And people who are drunk get behind the wheel of a car and kill themselves and other people. But we don't ban the responsible use of alcohol for that reason; we hold the drunk driver responsible even if he/she didn't crash their car. Same with other narcotics. The key here is that it is the individual user's responsibility, not the province of the state, to maintain an acceptable equilibrium.

I think that the use of illicit drugs is wrong. However i agree with spitfire88 on saying that if the person/user wants to consume those substances, then the government should not be able to deny the right to consume them.
However, if drugs were legalized, then there could be a huge increase in misuse and abuse. while your arguments are valid here is another viewpoint. the point of government is to protect the public. addicts will do lots of horrible things to get there fix. so making drugs illegal helps prevent addicts from hurting others to get their fix. just laws by themselves won't stop this. that is why there are police. still that in itself isn't the only reason. again on the same topic of giving up rights to ensure more rights for more people. Countries whether allies or not are constantly competing to be the best at everything. drugs simply put do not make you smarter.
The vast majority of illegal drugs were first developed to be used as genuine medicines. For example, cocaine was developed as an anaesthetic to be used for surgery. Over time, people realise the good effects these drugs can have and are likely to begin abusing them.

In my opinion, as long as the consumer is given full warning of the long term and short term effects of the product, it should be down to them to decide wether or not they should use it. If the drug is misused, then the consumer should be held resposible for their actions.