iTunes isn't a purchase mechanism as such, yes it let's you buy from apple, and it's the easyest route, but I don't use the Apple store much, and purchase other ways. To me the most important function of iTunes is that it allows me to organise my music and stuff, and backup my phone.
My Sony, Nokia 6210 and Blackberry phones all came with propriety software interfaces, as did my Palms, and I don't ever remember anyone else demanding to write interfaces for them, I doubt anyone tried. The only reason I can see that this is happening is that Apple became the biggest player through customer choice, and others wanted a slice, and think Apple should be obliged to let them have market share.
What do these people think Apple should have done, considering the up front risk they were taking. Even once successful, changing their market strategy would have been a very brave move even for Steve Jobs, hey investors, we are going to cut our margins, reduce our profits, and let competitors in, reducing the value of your stock.
It has always been, if you play with Apple you play with their Rules, if you don't like it, well there's always Android, Microsoft and Linux. We the consumer gave Apple their market, because we pretty much like them.