By any objective measure, iTunes has a monopoly with 87% market share in 2008. [1] Therefore, it can claimed that any actions against competitors can be construed as abusing monopoly power. Yes, there are other media players and other phones and other ways to buy media. However, the precedent is clear that Apple has "enough" of a monopoly to take these claims seriously. These claims are at least as serious as those launched against Microsoft in the 90's and in the EU-led accusations currently.
It's only a monopoly if there's a scarcity. There's no scarcity of music tracks; even if iTMS sells 99% of all tracks, unless they lock up the channel and prevent labels from selling to Amazon, there's nothing wrong with their dominant share.
I find the comparison between iTunes and Windows says more about the complainant than it does about Apple. Microsoft's monopoly prevented the development of competing technologies and enabled Microsoft to shut down entire sectors of the industry. Having the market's favorite music player doesn't prevent Amazon from selling music or Last.fm from streaming music or Microsoft from distributing Zunes.
[1]: http://news.cnet.com/8301-1023_3-10213149-93.html