It seems to me that many commentators are missing the distinction between a penalty and a tax in the same way that Chief Justice Roberts did. The error, grandiloquent as this sounds as I write it, derives from a fundamental misunderstanding of the nature of law.
Law has an inescapable moral component. Laws almost always have normative force. Let us leave aside the cases of laws that purport to command us to do something that is immoral, or to forbid us to do something morally obligatory. Those are unjust laws that are “no laws at all” in the sense of binding the conscience. Far more often, laws make that which was morally optional either morally mandatory or morally forbidden. There is no intrinsic moral reason that driving on the left side of the street should be impermissible. Lawmakers can nonetheless have good reasons for forbidding it, and once they have done so, their having done so becomes a good reason for us not to do it.
To my mind, what made the individual mandate so obnoxious was that it carried the law’s moral authority to a field where it was unnecessary and inappropriate. The government was purporting to bind our conscience in a new way: We were to be obligated to purchase a product, health insurance, as part of our general obligation to obey the law.