The Boston Bomber is an extreme example, but "black guy who caught 40 years on a weed charge" is an equally terrible characterization of the typical convict.
Here's a report on the prison population from 2017, which seems to be the most recent year available. Table 12 breaks down the offenses committed by folks in state prisons, which represents the overwhelming majority of prisoners. Table 14 does the same for federal prisons.
At the state level, most offenders (55%) are locked up for violent crimes. Another 17.5% were convicted of serious property crimes, like burglary. Drug possession only accounted for 3.5% of state prisoners. Drug offenses make up a good share of the federal prison population (47%), but that's almost all trafficking, not possession. I'm all for drug legalization, but most people who engage drug trafficking aren't the best people in the world, and I'd prefer that they not be allowed to vote. I'm definitely happy with murderers and rapists not voting.