“The federal government, as custodian of its prisoners, has the constitutional power to act in order to protect nearby (and other) communities from the danger such prisoners may pose,” says Justice Breyer.
The federal government can imprison sex offenders indefinitely after they have served their sentences (i.e. paid their debt to society) if they could be “sexually dangerous in the future.” The constitutionality of this decision is meaningless to me (not other people, though), because this is clearly a huge violation of human rights. I hope to update later with more links about the case.
Also, the Supreme Court barred life sentences for juveniles (unless they killed someone).