Unavailable Song Made Available After Purchase From Amazon

[Gordon_x64]

This isn't really a complaint or issue, I just want to know what happened. After an artists took one of their albums off spotify, I went to Amazon to buy a couple singles off of it. I downloaded the singles to my downloads folder, and now only those are back in my list. "Well thats just because it's a local file on your computer!" some would shout, but it's even back on my phones app, and shows the downloaded icon next to it(previously the entire song was gone). I had a bunch of my friends open the artist to see if they can see the songs I can, and they cannot. Again, I'm not complaining one bit, I'm actually pretty thrilled to get it back on my phone, but I am seeking to understand how purchasing it from a different service seemed to "unlock" it on my spotify account, and how I can do it with other now unavailable songs. Thanks!