I got this on another forum
"Those settings are for the "offline listening" downloads that are tied to your Amazon Music / Music HD player and streaming subscription. Cancel your streaming subscription and they're gone.
Purchased downloads that you can keep permanently come from the regular Amazon store, in MP3 only. From Amazon's perspective it's natural to cross-sell, but I think a lot of customers are going to be confused and unhappy
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so even though in Amazon you have the option to select HD downloads after you purchase a song, you only get an MP3 file when you do it