Although I fully expect Amazon to pull a LOT of songs from the Prime Music once the paid service gets fully established, I suspect what we are seeing right now in the trial paid period with the magical, mystical, disappearing songs is likely just sloppy work. Amazon likely rushed this to the table and now they might be finding that their worker bees occasionally added in the wrong songs or left out the right songs. Amazon is probably getting some nasty phone calls and letters from music industry lawyers telling them to pull songs they don't have the legal rights to include.