That's a good point. I did address the other levels here: https://news.ycombinator.com/item?id=23655174 but it was harder to find more of a breakdown in the data.

I don't have a link handy but it's around 15% of the total prison population who are in for drug offences.

I do have a link handy: https://www.prisonpolicy.org/reports/pie2020.html

Thanks! So between 10 & 20% are in for drugs -- the lower figure if I try to exclude trafficking (although roughly, as they don't always break it down). And this means "in for nothing harder than drugs" since they are counting by the worst conviction.