How the Supreme Court Upheld the Individual Mandate: It Is and Is Not a "Tax"

In Part II of his opinion upholding the Affordable Care Act, Chief Justice Roberts concluded that the individual mandate is not a "tax" within the meaning of the federal Anti-Injunction Act. However, in Part III-C of his opinion he found that the mandate is a "tax" for purposes of the General Welfare Clause. How could he reach both of those conclusions?

Chief Justice Roberts' Opinion in ACA Case - Introduction

The introductory portion of Chief Justice Robert's opinion reads like a primer on constitutional law. The Chief Justice lays out the basic structure of our constitutional system, including quotations from the two  foundation cases in constitutional law authored by the great Chief Justice John Marshall: Marbury v. Madison and McColloch v. Maryland.