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The author of the article misunderstands what monopoly means. He conflates monopoly with being a major success. A monopoly is when a company controls a particular market. Apple does not control the digital music market. They are merely successful in it.
Microsoft was/is a monopoly on the internet browser for Windows because it did not allow other browsers within its OS to really be used successfully.
On Macs, you can clearly use other music players and other music browsing devices to browse other music stores (e.g., Amazon's own music store).
iTunes itself is proprietary software with its own music store. It does mean it should allow other programmes or non-Apple devices to use it.
Get your conceptions about what monopoly means. These days, a lot of techies equate monopoly with the idea of being a major success in a market. Given that, then maybe one of the large American car companies like GM (or formerly) was a monopoly because it was the largest car company and it also wouldn't allow Porsche engines to be installed in their car bodies. (Yeah exactly. What???)