From TFA, the monopoly is on "after-market voice and data services" and applications for the iPhone.
The case law the judge cited was in regards to copy machines, namely Kodak using various illegal means to enforce a monopoly on replacement parts for Kodak copy machines. The Supreme Court ruled that the market for "Kodak copy machine replacement parts" was a different market than the market for "copy machines", and thus antitrust laws applied.
In this case, Apple has a monopoly on "iPhone applications" and AT&T has a monopoly on "voice and data service for the iPhone." If Apple allowed people to unlock their phones for use on other carriers, or to install apps not purchased through iTunes, no monopoly would exist.