Fourth, suppose we imagine a future world where the political process has adopted one of the seemingly silly purchase mandates. If so, we might question our easy supposition that it was so stupid; the very fact of enactment would mean our democratic process us had concluded otherwise. If the Supreme Court imposes its judgment that such a law would be undesirable, despite the lack of any constitutional basis, it will simply be allowing its preferences to trump democratic preferences.
Fifth, like any constitutional power, the Commerce Clause is subject to other constitutional limits. In particular, the constitutional right of liberty has been interpreted in a way that bans laws violating our bodily integrity. This means that even if Congress could make me buy broccoli, it cannot make me eat it. All it can do is make me pay money, a classically commercial act. Likewise, the health insurance mandate does not require anyone to actually undergo medical treatment. It just makes us pay money.
Sixth, because purchase mandates are just an obligation to pay money, they are really no different from taxes. Indeed, the challengers conceded that Congress could have imposed a financially identical requirement if it had just used the language of taxes and tax credits. Thus, the challengers themselves have no limiting principle that precludes any of their parade of horribles. Under their theory, Congress could still impose the dreaded broccoli mandate by just calling it a tax that one can avoid if one buys broccoli.
Seventh, while the challengers' argument relies on imaginary mandates that no one is even thinking of proposing, the parade of horribles on the other side is very real. Adopting the challengers' new principle banning federal purchase mandates would throw into doubt a long list of existing federal laws that mandate commercial transactions. One federal mandate requires corporations to hire independent auditors. Another requires that unions buy bonds to insure against officer fraud. Federal statutes also mandate that hotels and restaurants commercially deal with minorities and disabled persons. Federal antitrust law sometimes requires monopolists to supply their rivals. The list is endless. Are all such federal mandates now going to be the subject of new constitutional challenges?
The more fundamental problem with the challengers' method is that it asks judges to impose new constitutional limits based on their own policy preferences about how to treat various hypotheticals. This method is even worse than directly asking judges to create new limits based on their policy preferences, because it never confronts the question of whether the health insurance mandate itself is so clearly a bad policy. Instead, it invites the justices to create a new limit based on their policy preferences about hypothetical other laws like the broccoli mandate--laws that Congress is likely to never enact--and then applies that limit to laws like the health insurance mandate that are far less silly as a policy matter.
Worse, this whole "limiting principle" methodology itself has no limiting principle. One could take any Congressional power that is defined by existing doctrine and argue that the doctrine would have no limiting principle if Congress could use it to adopt stupid laws. Judges would have to limit Congress' power to prohibit commerce, because it could be used to adopt stupid prohibitions like a ban on broccoli or health insurance. Judges would have to limit Congress' power to tax, because it could be used to tax us all 110% of our income and then throw us all in jail when that proved impossible to pay. Judges would have to limit Congress' power to declare war, since it could be used to declare war on Bermuda if Congress didn't like Bermuda shorts.
The deepest problem with the challengers' method is thus the parade of horrible judicial decisions that would be unleashed by allowing judges to create new constitutional limits unsupported by constitutional text, history, or precedent in order to preclude imaginary laws no one wants to enact. I share the challengers' aversion to the "nanny state," but it would be far worse to replace our democracy with "nanny judges" who tell us which laws we can adopt.