Should offenders who abuse children be locked up for life, even after criminal sentence has been served?

Oprah's show today was about offenders who are kept locked away on an island in the state of Washington, even though their criminal sentences have already been served. They have already "paid their debt to society", and are being locked away in order to prevent them from committing any future crimes. As a mother of a young child, part of me is glad to see them locked away so they can't cause harm to any more children, but as an American who values freedom, I'm not sure I agree that it's OK to lock someone away in ANTICIPATION of them committing a crime. Your thoughts?