The reform points out that the national crime rate has increased based on a survey of police departments. But she never assumes that the number of crimes reported by each individual police department has also increased.

The reformer ignores the possibility that the crime rate might have been even higher if fewer people were imprisoned. In other words, maybe imprisoning more people actually helped keep the crime rate stable.

It doesn't matter whether the population increased significantly because the reformer’s argument addresses the rate of crimes and the percentage of the population in prison. The number of people that makes up the population is irrelevant.

The reformer’s argument only addresses the effectiveness of imprisonment on reducing crime. She doesn’t suggest any alternative measure for reducing crime, nor does she assume that other measures would be more effective. She just concludes that imprisonment is not effective.

The reformer never assumes that the number of prisoners and the number of crimes must be proportional. In fact, she doesn’t cite the actual number of prisoners or crimes at all.