I suspect this 5GB quote is a "fake", since there is no need to actually "copy" a song to the User's virtual drive, rather add a pointer to a copy already available on their platform, that is, if 1M users are buying a popular song, there is no need to clone this song million times, right?

Having said that, since there are smart people working on this platform on amazon, I am sure they don't make a physical clone, which raises the question already mentioned above, why limit to 5GB?