What mean this "incarceration"?

The AP reports this story on its wire:

Recent reports of three convicted killers escaping from supervised prison work programs prompted Gov. Bob Riley on Tuesday to bar murderers and other violent offenders from leaving the work release and community work centers to go to jobs.

I thought the purpose of incarcerating convicted murderers and violent criminals is to prevent them from moving about society and thereby protecting the public.

Why are these folks part of a work release program or participating in community work centers? And, if the report is accurate, why is Riley barring them only from leaving the centers? Why isn't he making sure they don't leave the prison?

Just another reason for death penalty advoctes to question the wisdom of sentences of lifetime imprisonment.