Preventative imprisonment?

Yesterday, the Supreme Court supported a provision of a law which allows "indefinite imprisonment" of sex offenders past any sentencing.

This seems like a hugely dangerous precedent, and in the article they note that North Carolina is already holding 77 inmates indefinitely under this provision.

Now, let me clearly say that I'm not pro-child molester, but I could easily see a law like this being expanded into other classes of crimes.

If you want life sentences for child molesting or any other crime, fine, pass a law putting life sentences on the crime. But this seems a very dangerous legal way to retroactively go about it.