It's the followup laws broadening the Sherman Antitrust Act that Apple violates. The amendments to the Sherman Act target anti-competitive behavior that results in market distortive effects, even in the absence of a monopoly. This includes price-fixing, collusion, and using leverage in one market to interfere with the market dynamics of another (usually smaller) market (see, e.g., Microsoft). There is at least one landmark case in which minor market participants were found to have violated the antitrust laws because their collusion would have anticompetitive effects even though their combined market share was less than half of the market, though the case dates to a more active time in antitrust jurisprudence.

Apple is no stranger to anti-competitive behavior; they are in fact the banner case for the last big antitrust case...which similarly did not involve a monopoly of any sort. (The book price-fixing case.)