The decision was 7-2, but the two dissenters agreed with the result; they fear the slippery slope.
"The Supreme Court said Monday that the federal government can keep "sexually dangerous" prisoners in custody past the completion of their sentences, overruling arguments that only states hold such power.
The vote was 7-2, with Justices Antonin Scalia and Clarence Thomas in dissent.
More than 20 states have laws permitting "civil commitment" of prisoners deemed sexually violent or otherwise mentally ill after they have served sentences for specific crimes. In 2006, Congress passed the Adam Walsh Child Protection and Safety Act, which authorizes the Justice Department to similarly hold federal prisoners who are nearing release or were found mentally incompetent for trial.
Justices Anthony Kennedy and Samuel Alito each wrote separately, endorsing the result but warning against carrying the majority's reasoning too far. "The Constitution does require the invalidation of congressional attempts to extend federal powers in some instances," Justice Kennedy wrote."