Hi jgrnt1,
The MusicNotes app just doesn't use the technology behind Scorch on iOS, yet. We haven't locked you out from some of your music on the Musicnotes app. Musicnotes have chosen to make their PDF content available to redownload on the iPad. Their use of Scorch on their website allows you to pay for a printed copy of a score, not the right to a digital copy.
There are a couple options that they could explore: either they approach us with the intent of using Scorch in their app, or they allow Scorch to have access to all their user account and sales information to make it possible for you to re-download the scores.
Either way, it's not possible now but anything's possible in the future
Sam