The guiding principle is this according to a report on statutory interpretation from the Congressional Research Service quoting a ruling in Alaska Airlines, Inc. v. Brock, 480 U.S. 678, 684 (1987) (quoting Buckley v. Valeo, 424 U.S. 1, 108 (1976)). "Unless it is evident that the Legislature would not have enacted those provisions which are within its power, independently of that which is not, the invalid part may be dropped if what is left is fully operative as a law." link
From the ruling:
4. CHIEF JUSTICE ROBERTS delivered the opinion of the Court with respect to Part III-C, concluding that the individual mandate may beupheld as within Congress's power under the Taxing Clause. Pp. 33-44.(a) The Affordable Care Act describes the "[s]hared responsibilitypayment" as a "penalty," not a "tax." That label is fatal to the application 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 healthinsurance; the payment is not limited to willful violations, as penalties 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 payment 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 unlawful. 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 insurance. See New York v. United States, 505 U. S. 144, 169-174. Pp. 35-40.