It means not spending resources keeping them alive and healthy in the already overcrowded and expensive prisons (assuming it didn't take years more worth of trials and mountains of legal paperwork, but that's another issue).
Just a side note - this is a problem much better addressed by reformation of incarceration laws. A lot of the burden on the prison system comes from jailing people who would be much better served with some form of rehabilitation program, notably the large number of jailed non-violent drug offenders who are the side effect of our badly-implemented "war on drugs". I'd hesitate to use scarce resources as an argument for the death penalty; likely that wouldn't be more than a blip on the Misspending Radar.
This might have something to do with the prison-industrial complex. Link
According to another Wikipedia article, most of these people are on probation or parole. I would also argue that many of the ideas behind modern rehab could be considered at least soft brainwashing.
The argument could the be made that, if our options are running a person's life out, or forcibly changing their minds, the latter is preferred. We don't have any pleasant options. The person, the person who is in jail, is the same person who committed a crime, and therefore can't be allowed out. The only options are to end the person as they are, or release them later simply because we can't afford to keep them there forever. Prison takes a life by degrees. Execution takes a life in a moment. Rehabilitation changes the person, yes, removing the person as they once were from existence, and substituting a similar. Given all options are equally immoral, or morally ambiguous enough to cause reasonable doubt, only the practical factor should be considered.
On a related note, California is currently far above capacity for their prisons, and a judge has ordered prisoners released if California does not address the problems of Prison Quality of life. They currently have to find a way to be rid of over 10,000 inmates, at this point likely through shuffling them out of state and prison reforms.