The lyrics are copyrighted, so books with lyrics cost a lot of money. Melodies are the same.
What you bought is a BEAUTIFUL app for:
- Making your own charts (or modifying the ones you download from the forums)
- playing those charts with a number of settings (styles, tempos, repeats, looping...).
The app is exceptionally well done and thought-out, but the price is low because of copyright concerns, and objectives. One can always get the melodies elsewhere...