Apple didn't voluntarily stop using DRM; they convinced the labels to stop requiring it. Piracy isn't Apple's concern, but the user experience is.
This is in service of saying that DRM isn't in the user's best interest and even big companies that care about user experience don't like DRM. Apple didn't ship DRM on its OSes when they were DVD based either. Yes they probably lost some sales due to piracy, but not having to deal with serial numbers is big for users.
No, Amazon stopped using DRM to have a competitive advantage versus Apple, the music companies went along because it gave them a real competitor to iTunes, and eventually Apple managed to renegotiate their contracts so they could also remove the DRM.
Are you sure it wasn't the other way around? People were building huge libraries of music that only worked on Apple products (iPods and iTunes) around the time the labels allowed Amazon to start selling DRM-free MP3s. Apple would have gained an enormous amount of leverage if the labels had allowed that to continue: "Oh, you won't accept our terms? Well, then your product won't be playable on 90% [0] of portable music devices [1] and won't be available in the largest online music store (and your competitors' will be)! I hope not all your artists have left by next fiscal quarter." For all he talked amount it, I'm not convinced that Steve Jobs ever wanted to see the iTunes Music Store go DRM-free.
The market penetration of iPods meant that only path forward for the labels was one that allowed people to listen to music on their iPods. The only way to do that was selling DRM-free MP3s or protected AAC files. As a bonus, DRM-free MP3s worked on all the cheap flash-based players (which always had greater deployment than any of the DRM-compatible players from Sony and Microsoft).
[0] A number generated by my PRNG (posterior random number generator). But the iPod Classic really does have an absurd share of the >64GB market (probably declining now that you can get tablets and phones with 128GB of flash).
[1] Don't forget that Apple has only ever supported their own DRM. Protected AAC files play on Apple devices and only Apple devices. Apple devices support only protected AAC and DRM-free formats.
This is in service of saying that DRM isn't in the user's best interest and even big companies that care about user experience don't like DRM. Apple didn't ship DRM on its OSes when they were DVD based either. Yes they probably lost some sales due to piracy, but not having to deal with serial numbers is big for users.