In resolving [this] question, we assume, but we do not decide, that other provisions of the Constitution—such as the Due Process Clause—do not prohibit civil commitment in these circumstances.Cf. Hendricks, 521 U. S. 346; Addington v. Texas,441 U. S. 418 (1979). In other words, we assume for argument’s sake that the Federal Constitution would permit a State to enact this statute, and we ask solely whether the Federal Government, exercising its enumerated powers, may enact such a statute as well.
I suppose we should be grateful that Breyer et al didn't "decide," but their assumption is as good as a decision until someone challenges the indefinite-confinement rule on due-process grounds. As the majority insists (and as was argued by the Solicitor General, i.e. Justice-designate Kagan), this case was focused narrowly on whether the rule trespassed on state rights. Its real test will come when its trespass on human rights is challenged.
The majority also argues from the precedent of federal commitment of the mentally ill, and that approach makes me worry about future diagnoses of mental illness in "dangerous" prisoners of other kinds who will have otherwise paid their debts to society. But if society determines that pedophiles are a perpetual danger to the general public the proper remedy should be life without parole for the first offense -- or else the diagnosis of sexually dangerous mental illness should be made at the sentencing, not after. The alternative endorsed by the Court gives the government too much discretion without proportionate due process. It's one case in which the classic cliche of the "slippery slope" seems appropriate. Thomas and Scalia are reactionaries, but sometimes their hypersensitive hostility to progress can serve a canary-in-the-mineshaft function. Just as paranoids may have real enemies, reactionaries my sometimes react to something genuinely dangerous. Maybe someone can reassure me about this case, but it doesn't sound like a good idea at all in the long term.
And what's a lettre de cachet? Look it up here.