The Amazon MP3 store is now optimized for iOS devices….[so now], you can purchase music from Amazon’s catalog…directly from an iPhone or iPod touch…. Additionally, purchases will be available for immediate streaming and download from within Amazon’s Cloud Player iOS app.
This is Amazon fleshing out their coverage of user activities—with their ability to give you access to your already-purchased content from so many different hardware-tied ecosystems (iOS, Android, PC/Mac) already, they had a pretty thorough coverage of the user-focused goal of "my content, anywhere, from any device" (e.g., their Kindle, Cloud Player, and their respective apps). This move adds the coverage of the user activity "adding to my content" in a way that makes it immediately accessible—again, anywhere, from any device. That coverage of most of most users’ music and other content activities is a key part of iTunes’ success.
Along with their recent AutoRip feature, which puts any AutoRip-enabled CDs that you purchase or have purchased from them going back to 1998 automatically, instantly in your Cloud Player, Amazon’s music buying options may well be the fastest way to make your music purchases available "at any time, from any place, no transferring or synching–your music, everywhere" (from one of Amazon’s Cloud player videos).
More of what most users will want to do with Amazon’s content will now be even more platform-independent. This is Amazon building out their their content-delivery and storage ecosystem to out-flank all the hardware-tied ecosystems’ obstacles, offering those users a way to loosen those other ecosystems’ hold on them.
What could be next goals for Amazon in this vein? Maybe giving that same freedom to the other content that they sell: books, hopefully in various bundles with their ebook and Audible audiobook versions, and movies (offering bundles or just straight bundling streaming rights or digital copies with purchases of DVDs and Blu-ray discs).
Freeing users of arbitrary constraints? That’s a good thing.