Jim, the "90%" claim is purely quantitative ie that a percentage of songs in oral tradition can be found in print. I don't have the book in front of me, but I don't recall it claiming that these were all by professional composers (although many of them were).

Whatever the origin of a song, singers and their audiences would only take it up if they felt it related to them in some way. Whether that is because it came from someone with direct experience of the subject or from an imaginative professional songwriter, once it had been taken up and turned into folk song it became the voice of the people. If the sentiments of a song expressed their own feelings, why should it matter who originally composed it? It almost certainly didn't to the singers, or the songs wouldn't have entered the tradition and been passed on.