I'll just quote the first sentence from that article:
"Despite intensified competition from fierce rivals including Microsoft Corp. and Wal-Mart Stores Inc., Apple Inc.’s iTunes store in 2010 managed to hold onto its dominance in the U.S. market for movie electronic sell through (EST) and Internet video on demand (iVOD), new IHS Screen Digest research shows."
Wait, one more quote from the 3rd paragraph:
“The iTunes online store showed remarkable competitive resilience last year in the U.S. EST/iVOD movie business, staving off a growing field of tough challengers while keeping pace with an dramatic expansion for the overall market,” said Arash Amel, research director, digital media, for IHS. “Apple faced serious competition from Microsoft's Zune Video and Sony Corp.'s PlayStation Store, as well as from Amazon and—most significantly—Wal-Mart."
Remember, when you're talking about a monopoly, you're talking about competition in the market. If iTunes has managed to attract a majority of consumers in a market rife with competition, then one can only cry monopoly after digging up anti-competitive practices that specifically play a larger factor than normal consumer choice.