You can download an MP3, but you still don't "own" it in the conventional sense. You are licensing it, and a downloaded MP3 is how you utilize that license. If that license was revoked for some reason (admittedly not a likely scenario), continuing to have the MP3 on your system would be a violation.
In practice I doubt this would ever be an issue, but just wanted to point out that you effectively never "own" a digital reproduction of something unless you are the actual copyright holder (or the copyright is permissive), and digital copies can be clawed back in a way that a CD or physical book you purchased cannot.
So long as the copy is DRM-free, which I’m pretty certain Amazon and Apple are these days, there is no means to enforce such a revocation. Of course, you could find yourself unable to download a new copy should you ever need to.
I believe there have been instances where Amazon removed books from user Kindles, which (thought these files are indeed DRM'd), that is not the mechanism by which this was done.
"If I'm willing to violate the law I'm good" is true and I don't disagree, but that also applies to full-blown piracy.
My understanding is that it’s basically the same as ripping a CD - I have a perpetual license for personal use.
Of course if I broadcast it publicly or share it on bittorrent I am in violation. But if all I do is keep it in my music library for myself to listen to, it’s OK.
So, while the MP3 is covered by an Amazon license and the ripped CD has an implied fair-use license, those license terms are more-or-less the same.
> digital copies can be clawed back in a way that a CD or physical book you purchased cannot
There is no difference between an mp3 file and a CD? A CD is exactly a digital copy (a book is an analog copy). What's true for one has to be true for the other.
Nope. You own the CD. You do not own the MP3.