When people register as a sex offender the police do monitor them, check to make sure they haven't moved etc. They are not allowed to live near schools etc. Their names, pictures, and addresses are published publicly so the community and schools know who they are.
When they move into a neighborhood you are notified, regardless of whether they are on parole or not. Giving parents and other adults a chance to protect children from predators.
Please quit with the lawyeresque posts.