...again, (or one more time, if you will)
This is not about the ability for sex offenders to be held indefinitely. This is about Congress' power to make statutes that apply among all the states over such issues. Breyer's opinion specifically stated that this holding makes no determination whether it is substantively unconstitutional on other grounds.
If you want to bitch about the policy, bitch at Congress for passing the law in the first place. Save your salvos for when the SCOTUS rules on its 5th and 8th Amendment merits. It'll most likely be overturned on those grounds 6-3, with Roberts, Alito and Kennedy dissenting, and Scalia and Thomas concurring on the judgment, but not the opinion.