The challenge in this case was not whether the federal government had the right to indefinitely incarcerate "sexually dangerous" people, but whether Congress had the right to overrule the state's right to do so.
Respondents’ contention that §4248 violates the Tenth Amendment because it invades the province of state sovereignty in an area typically left to state control is rejected. That Amendment does not "reserve to the States" those powers that are "delegated to the United States by the Constitution," including the powers delegated by the Necessary and Proper Clause.
Furthermore, this did not create some slippery slope, unless your disagreed with he original Congressional statute to begin with. It is equally clear people did not follow the link and actually read the decision. Check out the bold phrases in the part of the decision listed below.
Section 4248 differs from earlier statutes in that it focuses directly upon persons who, due to a mental illness, are sexually dangerous. Many of these individuals, however, were likely already subject to civil commitment under §4246, which, since 1949, has authorized the post-sentence detention of federal prisoners who suffer from a mental illness and who are thereby dangerous (whether sexually or otherwise). The similarities between §4246 and §4248 demonstrate that the latter is a modest addition to a longstanding federal statutory framework. Pp. 9–14.
The claimant ruled against in this decision did not assert they were illegally sentenced or held. No due process or other Constitutional rights, i.e. Amendments 1 through 9, were invoked.
The Court does not reach or decide any claim that the statute or its application denies equal protection, procedural or substantive due process, or any other constitutional rights. Respondents are free to pursue those claims on remand, and any others they have preserved. P. 22.
The case rested entirely on the federal government's right to act in a necessary and proper fashion to protect the public, even in cases where states are unwilling or unable to do so. Had the majority ruled otherwise, and it is not surprising the liberal justices did not, it opens up a huge can of worms involving the right of the federal government to protect the public from just about anything. What happens to Climate Change? Can't states just claim the federal government doesn't have the right to regulate the activity in the interest of protecting the public. That's the state's job, right? Other things, such as work-place safety, food safety, protection from pollution, acid rain, deadly levels of lead, etc. All of these things would have been at risk had they ruled any other way.
This ruling did not give the government any additional powers to incarcerate people, it merely reiterated the right of the government to protect the American people even when states are unwilling or unable to do so. People really need to read the ruling. Anybody who disagrees should justify why the federal government doesn't have the right to protect the people when states don't want to.