Where does you library go when you change devices? Aren't tunes licensed to a particular device? Which would make it different from Kindle for instance, where your license is in the cloud, and content accessible from any authenticated device.
iTunes: m4a, has your name and email address encoded
Amazon: mp3, has Amazon's unique song id in the ID3 comments
And in terms of the cloud:
iTunes: $25/yr for iTunes Match, which is completely optional
Amazon: Free storage for songs you purchase from Amazon + 5gb extra
The important issue is to keep your music backed up, since both Amazon and iTunes offer limited options if your harddrive fails.
Anyway, to address your point, Dropbox or a manual file transfer would be your best bet for multi-device support.
How common is this though? I've half-heartedly done a bit of research from time to time about Kindle DRM, but always give up after (perhaps incorrectly) confirming that it is still mostly the default, and after finding pages and pages of results of blog posts bemoaning or defending some DRM related news from last year, or two years ago, or four years ago. It's why I, like many others in this thread, still buy print even though I'd consider a Kindle for convenience.
So if Kindle books without DRM are common, or even the majority, I'd genuinely like to know.
> Just as with iTunes, people should stop repeating what was once true, and focus more on what is true.
I'm probably guilty of repeating anti-DRM complaints regarding the Kindle, but you have to admit that with iTunes and Amazon music, it's clear cut and there's no question when it comes to DRM.
The magic phrase "At the publisher's request, this title is being sold without Digital Rights Management software (DRM) applied." is usually added to the product description.