Last I knew, it was justified under public safety. People who had already been sentenced and served their time in the past had been listed on newly-created sex

Last I knew, it was justified under public safety. People who had already been sentenced and served their time in the past had been listed on newly-created sex offender lists. Court ruled that it wasn't a punishment, so it couldn't be considered ex post facto punishment. Wikipedia indicates the case that decided this was Smith v. Doe, 538 U.S. 84 (2003).