Just put aside that, is it actually any legal reason for using things like dropbox to upload music purchased and streamed for private uses?
BTW that one album/20GB offers only last until the end of the year, and it only last for a year. After that, you would have the habit of using the service but be reverted to the free plan.
However since all the new purchases on amazon are automatically saved without counting towards the quota, I guess that solves the problem for many android users. Just that I don't listen to many of bands/artists that put their songs on Amazon MP3 may make this offer less appealing.