I cant think of why HMX would get out of the business of charting songs, since it is one of those items that defines HMX's games. If you look on the forums, there are a number of threads on how much better the charting is as opposed to GH, for example.
It costs money to chart the songs, bands would want some cut of the royalties as would HMX, the Console of choice among others.
I would think that when HMX decides to do a song or album, they pay the band/label etc for the rights to do this. HMX is also looking at the bottom line and are they going to "Make" money after they pay all of their up front costs out over time based on sales.
How does HMX make money over time if A. no one buys the songs to the level that is needed or B. The song is given away for free?