What exactly about the iTunes store does she like more? I'm just wondering what causes some people to prefer the iTunes store. Is it just that she uses iTunes and it is "just there", so she's never really gone elsewhere? Or has she tried the Amazon store, and not liked it?
For the non-technically inclined iTunes makes the process a no brainer. Easy to find and purchase what you want, and then the tracks are 'just there' when you are done with the purchase (nothing else do to get the music in your library). That and the fact that it is built in to iTunes puts it 'in your face' when working with the library.
I am not saying that Amazon is hard to use but:
#1 Just pulling the stuff up in your music program (iTunes on her Mac) and buying it right there and having it add to your library automatically vs:
#2 Open up a web browser, find and buy the music, then manually add the music to your library
#2 is not THAT much harder, but for someone who is non-technical and just wants it to be easy and work #1 wins out because it is easier and already in front of you.
I have explained to her the issues with DRM, but that is one of those things that most people just don't get until they are bit by it - and that won't happen until the first time she wants to use the tracks outside of the iTunes/Apple infrastructure.