Back in those days, before sentencing guidelines, the U.S. had parole and after a certain number of years, you maxed out your sentence and had to be released. For a life sentence, it was 30 years.
From Wikipedia:
A native of Charleston, West Virginia, she was a former nursing school student, Women's Army Corps recruit, and accountant. Moore had married and divorced five times and had four children before she turned to revolutionary politics in 1975.[4]
Moore's friends said she had a deep fascination and obsession with Patty Hearst. After Hearst was kidnapped by the Symbionese Liberation Army, her father Randolph Hearst created the organization People in Need (P.I.N.) to feed the poor, in order to answer S.L.A. claims that the elder Hearst was "committing 'crimes' against 'the people.'" Moore was a bookkeeper for P.I.N. and an FBI informant[6] when she attempted to assassinate Ford.
She had a great response to Matt Lauer's question about people who think she should not have been released:
As for those who say she should never have been released, Moore says: “We have a Constitution and we have laws. Regardless of who you are, there were conditions to be met for me to be paroled, and I met those conditions.
“If people object to that, write your congressman and ask that your law be changed.”
She is still being supervised on parole.
Leslie Van Houten comes up for parole for the 19th time this year. She will be 60 and have served 40 years of her state life sentence. There's no mandatory release for her. As I wrote in 2002, I think it's time for her to be freed.