Scalia and Thomas, who tend to judge with the original intent of the Constitution in view (a position with far more integrity than many well-meaning folks appreciate). Thomas contends that this bill gives the federal government authority not granted in the Constitution.
Though the bill seems narrowly tailored, my initial take is that it further muddies the already murky law concerning sex offenders. More worrisome is that though right now the application is narrow, the authority recognized is broad and spells danger for just sentencing procedures. And much more worrisome is that it seems to be another instance of the Court rendering the Constitution as only a floor for the federal government, though it was meant to be a ceiling, too.