Well one way to keep them in is if they attacked a guard (supposedly) or disobeyed rules (supposedly) so that more years could be added to their sentence.
Michael Swanson, the teen who murdered two clerks at two different stores in Iowa in cold blood after they gave him the money he demanded, will at least remain in prison for 60 years before being eligible for parole. He also tried to kill another inmate while incarcerated, so that should get him a few more decades.
He’s the one who laughed and displayed a broad smile throughout his arrest and trial as the details of the murders were being recounted.