Distributable material means that they would have to pay royalties. In as far as a financial decision, I don't know this for sure but I assume it would be more cost. My thought is that offering songs at all would then be less feasible.

I also think that since so many songs have multiple parts that it would be hard to make a backing track that removes the part you want.

Reality is, this is teaching site much in the way a personal teacher at store would be. From that perspective, I guess I wouldn't have thought that personalized content such as downloadable tracks would have been part of the package. They are trying to teach you how to play. What you do with it from there is your prerogative.