I actually don't know off the top of my head about the federal system, but in terms of state prisons, especially the states I've lived in (CT and CA mostly), yes, the majority of people serving time for drug law violations are repeat offenders of simple possession or people whom the law deemed as traffickers or dealer because they had a certain amount that crossed some largely arbitrary threshold established by the law.
On another note pmoseman, I am genuinely happy that there is a BLer around here who isn't only quite intelligent but also knows something about the legal system in the US, despite how fucked up it is (and doesn't just focus on how fucked up it is; despite how fucked up it is, I'd rather have to deal with the rule of law US style than Thai style - although I'd rather deal with Thai style healthcare than US style healthcare, but I digress). No joke, even if we don't always see eye-to-eye (although I think we mostly do), it's refreshing to have a different/new and more to the point thoughtfully critical point of view around here.
Perhaps I should have said that the majority of people involved in the criminal justice system (and it's a large system as I hope we'd all know by know) are so involved because of drug law violations (indirectly or directly; or violations of related laws/policies/prohibition/etc). For instance, in CT, most people in prisons in the state itself (and in the prisons outside the state the state contracts it's prisoners out to house) are there for drug law violations, and the majority of people involved with the criminal justice system in CT are likewise involved because of drug law violations. Ugh, I feel like I'm repeating myself now so I'll shut up for a minute.