Unfortunately drug addiction does not stay isolated to a fixed dosage at a fixed price. It becomes ever more addictive and then the spiral starts as th habit needs to be fed. This leads to crime and violence when the money runs out to feed the habit. This is when the rest of society starts to suffer the effects.
Drugs have a far reaching spider of influence that is dark and nasty. I care little if an addict kills themselves as its a individuals right to chose stupid or not. But it's not isolated to their own personal demons it effects the rest of society.