Yet another bonehead move by Apple. Let's take a great program and wreck it, piece by piece, over the course of a decade.
That said, I have a solution to this problem. Depending on how much purchased music you have in your library, it may be tenable or it may not be worth it for you. To fix the issue, select a song that you have purchased from iTunes. Left-click with your mouse to find this option, or go to File > Create New Version > Create ACC Version. This will make a duplicate of your file and it will keep all of your metadata intact with one exception. The "kind" field will change from "Purchased AAC audio file" to "AAC audio file."
iTunes will no longer recognize this song as a purchase, and, once all of the purchased songs are eliminated from each artist's catalog within your library, the Columns of Stupidity and Shame (COSS) will disappear.
Restarts may be necessary.
If anyone has a more simple solution, I'd love to here it. But, right now, I have 460 songs to deal with.
Good luck, folks.
N.