Of course drug dealers don't want drugs legalized. DPR claimed he wasn't just a simple dealer but rather some ideological crusader for a better tomorrow (by which he meant: anarcho-capitalism, his idea of utopia). He wasn't only selling drugs remember, but also malware, forged documents, he experimented with selling weaponry, etc ...
While I would question DPR for arguing against legalization (if he indeed said that) on the basis of some "holy crusade" against narcs everywhere, ultimately, his efforts to bring a viable online drug market to the masses have been highly successful (until today), and prove that such a possibility can be a reality. As long as you don't post real-identity email addresses, that is.
And what's wrong with forged documents? Or weapons?