I think it lies in the area of "humanely" killing. Street drugs, such as heroin, do not provide a "humane" death when overdosing. It's not clean, and there is something incredibly twisted about using a "recreational" (albeit highly illegal) drug to put a prisoner to death. It's got to be separated in order to be sanctioned.

Using lethal injection used to be a fairly "clean" way of doing the deed; there was no pain, no unsightly convulsions, no obvious distress. I suspect it has to do with the people involved in this last act as much as it does the prisoner's rights...but I am just guessing.