With regards to SR the other side of the coin (no pun intended) is that allowing consumers to buy their illicit substances online is a comparatively safer and cheaper option then the existing unregulated street activity - especially with the rating systems online marketplaces provide.
It also stands to reason that by moving online it cuts out a lot of the middlemen and associated street level violence; when everyones 'block' is the world - it likely makes drive-by's that much harder...
So true, politicians are mostly concerned about the visible drug trade and crime. The well being and safty of drug users it not an argument you will hear in a political discussion.
But you could argue the same about child pornography. It's quite "invisible" and there is no direct relation between it's popularity, if you can call it that, and the number of offences against children.
You can't argue as a politician that we are cracking down on child pornography because it will measurable reduce the number of cases of child abuse in our country. However there is a relation, the pictures have been taken yesterday or ten years ago. The strongest argument against child pornography is that it normalise and promote abuse of children. That is why Cartoons or other artistic renderings of non existing children in degrading and sexual scenes also are considered child pornography in most countries.
So the political argument against buying drugs online would be that it lures people into drugs by being easy obtainable on not stigmatising as you don't have to seek shady characters on street corners.
Another point would be that there are a large selection of more dangerous drugs available on line, that your street corner dealer doesn't carry.
It would, as you say, break the monopoly held by local gangs in the street and possible reduce the number of drive by's. But organised crime wil not go away it will go on line instead. Silk Road might be run by mobsters, we don't know, it would be a clever way to maintain a large network of dealers and distributors without them even knowing who they worked for.
