Best known for establishing a national sex offender registry (of highly questionable fairness and effectiveness) and promulgating standards for state registries, the Adam Walsh Act also provided for the indefinite "civil commitment" of federal prisoners who have completed their sentences but are considered likely to commit sex offenses if released. People who trust the federal bureaucracy and believe officials will use this power accurately, fairly, and in good faith may feel protected by it; others should worry about a government authorized to detain its citizens indefinitely, without jury trials, based on speculations about their future dangerousness.
Who qualifies as "sexually dangerous" under this statute? By what process are people indefinitely committed? Virtually any federal prisoner, including ones who have not been imprisoned for sex offenses, may be targeted for civil commitment. (Since convictions "for violent and non-violent sex offenses constitute less than 2 percent of all federal convictions," the National Association of Criminal Defense Attorneys stresses the statute provides for "potential commitment of a significant number of persons with no criminal history of sexual misconduct" and even persons with no prior convictions for any crimes, like material witnesses being detained by the attorney general.) Once targeted, people are entitled to a hearing before a federal judge (or magistrate), but their alleged future dangerousness need not be proved beyond a reasonable doubt. And, while the judge must find that the prisoner "has engaged or attempted to engage in sexually violent conduct or child molestation," that finding (as Justice Thomas noted in dissent) "can be established by ... clear and convincing evidence that the person committed a sex crime for which he was never charged." In other words, the government is empowered to imprison (or "civilly" commit) people indefinitely without bothering to try them in criminal court or in any other proceeding in which they are afforded the rights of a defendant in a criminal case, even if they have never been convicted of any sex crimes.
The wide applicability of this statute and lack of procedural safeguards distinguish it from state civil commitment laws previously upheld by the Supreme Court (this argument is presented in convincing detail in the NACDL amicus brief). In 1997, in Kansas v. Hendricks, the court upheld a controversial Kansas statute providing for the civil commitment of people deemed likely to commit "predatory acts of sexual violence" due to "mental abnormalities" or "personality disorders." But at least the Kansas law applied only to people previously convicted or charged with specified sex offenses and provided for jury trials in which alleged future dangerousness would have to be proved beyond a reasonable doubt.
Hendricks upheld the Kansas civil commitment scheme by a five-to-four vote, in a conventional liberal-conservative split. Justice Thomas wrote the majority opinion upholding the law, joined by Justices Rehnquist, Scalia, O'Connor, and Kennedy; Justice Breyer wrote the dissent, joined by Justices Ginsberg, Souter, and Stevens. In Comstock, however, the majority and dissent switched sides. Justice Breyer wrote for the majority, upholding a federal civil commitment statute that's less protective of individual rights than the state statute he would have struck down (he was joined by Ginsberg, Stevens, and Sotomayor; Kennedy, Roberts, and Alito concurred). Justice Thomas wrote the dissent, joined by Scalia.
What accounts for this switch? In Comstock, the court didn't address due process challenges to the federal civil commitment scheme; it was reviewing a decision by the Fourth Circuit Court of Appeals that struck down the federal statute as an unconstitutional exercise of congressional power, without reaching questions about due process (or other individual rights). So, putting individual rights issues aside, Justice Breyer adopted the expansive view of federal authority advocated by Solicitor General Kagan. (At the Volokh Conspiracy, Orin Kerr described Kagan's position as "shockingly broad: She argued that the Constitution gives the federal government the general power 'to run a responsible criminal justice system,' and that anything Congress plausibly thought a part of running a 'responsible criminal justice system' was within the scope of federal power.") Justices Thomas and Scalia, in keeping with a more literal, or originalist, reading of Article I congressional authority, and a view of the federal government as one of explicitly enumerated powers, would have denied the federal government civil commitment powers similar to those they afforded the states.
So there is some logic to these oddly contrasting positions represented by Justices Breyer and Thomas in their reviews of state and federal civil commitment statutes, respectively. Technical, constitutional arguments aside, however, I do wonder how Justices Thomas and Scalia would explain, in lay terms, why they oppose federal power to detain people deemed "sexually dangerous" in civil commitment proceedings but support federal power to criminalize mere possession or even false advertising of child porn, imposing long sentences on people who have never committed or even been suspected of committing any actual sexual offenses.
In any case, the court may get another chance to review the federal civil commitment provisions of the Adam Walsh Act. The Comstock case will return to the Fourth Circuit, where, as Justice Breyer observed, Graydon Earl Comstock (et al) may raise due process, equal protection claims or any other individual rights claims on which the court has not yet ruled. But even if the case eventually wends its way back to the Supreme Court, and even if Justice Breyer switches sides again and somehow takes four of his colleagues with him, much damage has been done. Federal criminal jurisdiction has expanded dramatically in the last few decades, with the blessings of conservatives and liberals alike. Now, thanks to the ruling in U.S. v Comstock, that power seems practically infinite: federal authorities can imprison people indefinitely on suspicions of future dangerousness. It's not just suspected sex offenders or terrorists who are at risk.