Even if we assume the validity of such laws, Justice Clarence Thomas notes in a dissent joined by Justice Antonin Scalia, civil commitment does not carry them into execution, as required by the Necessary and Proper Clause. The criminal statute has been fully executed at the point where someone convicted of violating it completes the sentence it prescribes, which is precisely when civil commitment takes effect.
Continued confinement, Thomas notes, "is aimed at protecting society from acts of sexual violence," not at "'carrying into execution' any enumerated power." That point is reinforced by the fact that one-fifth of the prisoners whom the government has identified as "sexually dangerous" were never convicted of a federal crime involving sexual violence. Even someone convicted of mail fraud or tax evasion could be put in this category.
Furthermore, Thomas writes, "the definition of a 'sexually dangerous person' ... does not require the court to find that the person is likely to violate a law executing an enumerated power in the future." The commitment law therefore is only tenuously related to federal criminal statutes, which themselves may be only tenuously related to an enumerated power.
Thomas warns that the majority's opinion, which requires no more than a "rational" connection between a federal law and an enumerated power, "comes perilously close to transforming the Necessary and Proper Clause" into a rationale for the general police power that the Constitution reserves to the states. "The Constitution does not vest in Congress the authority to protect society from every bad act that might befall it," he writes. Unfortunately, this will be news to most members of Congress.