I'll post an excerpt from the opinion that Roberts wrote, in which he explains it was clearly not *intended* to be a tax but still is for the purposes of the Constitution:
4. CHIEF JUSTICE ROBERTS delivered the opinion of the Court with respect to Part III–C, concluding that the individual mandate may be upheld as within Congress’s power under the Taxing Clause. Pp. 33– 44.
(a) The Affordable Care Act describes the “{s}hared responsibility payment” as a “penalty,” not a “tax.” That label is fatal to the appli- cation of the Anti-Injunction Act. It does not, however, control whether an exaction is within Congress’s power to tax. In answering that constitutional question, this Court follows a functional approach, “{d}isregarding the designation of the exaction, and viewing its substance and application.” United States v. Constantine, 296 U. S. 287, 294. Pp. 33–35.
(b) Such an analysis suggests that the shared responsibility payment may for constitutional purposes be considered a tax. The payment is not so high that there is really no choice but to buy health insurance; the payment is not limited to willful violations, as penal- ties for unlawful acts often are; and the payment is collected solely by the IRS through the normal means of taxation. Cf. Bailey v. Drexel Furniture Co., 259 U. S. 20, 36–37. None of this is to say that pay- ment is not intended to induce the purchase of health insurance. But the mandate need not be read to declare that failing to do so is un- lawful. Neither the Affordable Care Act nor any other law attaches negative legal consequences to not buying health insurance, beyond requiring a payment to the IRS. And Congress’s choice of language— stating that individuals “shall” obtain insurance or pay a “penalty”— does not require reading §5000A as punishing unlawful conduct. It may also be read as imposing a tax on those who go without insur- ance. See New York v. United States, 505 U. S. 144, 169–174. Pp. 35–40.
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