The claim authority by way of that steaming pile of socialist sophistry referred to in polite company as the "New Deal Commerce Clause". Basically they claim the right to regulate anything that has a "substantial effect on interstate commerce".
They do the same for the DEA and EPA, so they're getting ready to ban posession of guns like they banned posession of marijuana. How's that been working out for them?
Pretty good, actually. The Raich decision by the supreme court illustrates it perfectly: the court claimed that the drug-prohibition laws were legitimate under the commerce-clause -- even in the case of home-grown marijuana, which was not sold inter- or intra-state -- because the lack of a market [the substance being illegal] did impact commerce, because if it did exist then it would.
That is the sort of non-logic the courts [and other government branches] have been shoving at us for years.
The Obamacare decision, contrary to what some believe, did not set any limit on the "commerce clause" but instead opened "taxation" up to be nothing more than extortion, stripping it of the last vestiges of legitimacy that it had:
The obtaining of property from another induced by wrongful use of actual or threatened force, violence, or fear, or under color of official right.
Under the Common Law, extortion is a misdemeanor consisting of an unlawful taking of money by a government officer. It is an oppressive misuse of the power with which the law clothes a public officer.
Source: legal-dictionary.thefreedictionary.com