How many times do you hear of people getting kild over "alcohol wars" or "tobacco wars" on the street? Any vodka cartels that you know of? I can't think of any.
I don't know how things are in the States, but here in the UK several drugs are controlled and taxed by the government. Those legal drugs include tobacco and alcohol. Because they are taxed, there still exists a black market for untaxed drugs which are cheaper for the consumer. People inevitably get killed as a result of this.
Example
The difference is at the local businessman level. As it stands, there are no legal protections for the salesman of the illegal drugs. His business concerns are managed purely through market agreements of price and patch etc. If a well funded competitor was to break market agreements and start undercutting another's business - the salesman under attack cannot complain to the monopolies commission. The only recourse is by legislating themselves in a 'black judiciary' of vendettas.
Such things spiral out of control and we get the street level violence we are aware of.
If drugs were legal, there would still be a booming black market trade, just like with alcohol and tobacco now - but it will pale into insignificance in comparison to the legitimate trade and the original black market trade.
And of course - the taxes from the legitimate business can be put into increasing funds for cracking down on the black market to make sure that the business is a risky one to be in as a potential deterrent for those considering making quick and easy money (though as with the example above, large legitimate businesses are often involved directly in their black market equivalent).