This content is almost entirely their own, and they legitimately want some of the profit.
I don't buy it. The content is a licensed derivative work and the cost of the conversion is entirely on the Rockband/Guitar Hero crew. The receivable content that the consumer cares about is the datafile with the button clickies. All any particular song does is increase to slightly increase the market penetration of the core product. In Hollywood, if someone options your book for movie rights at 10% you feel lucky and have a party. That they want more than that for doing no work is myopic greed. No more, no less.
Honestly, If I ran the RockBand/Guitar Hero crew, I would cut out the music industry entirely. The datafile with the button clickies is only a derivative work because they include both the MP3 and the button clicky data in the same file. If they split the file format, they could say, "Here buy our datafile to play your previously purchased MP3's in RockBand!" The RIAA would then only get whatever pittance they can get from iTunes or Amazon. Rock Band/Guitar Hero could keep the entire profit for themselves, completely legally. I would bet that the only reason they don't do this now is that they don't want them to walk away entirely prohibiting Rock Band/Guitar Hero from being able to purchase the songs for the initial game disc. That and the technology for splitting out instruments still has a bit of a ways to go so being able to license the original masters does help.
But there isn't anything understandable about their complaints. It is just raw, short term greed with no focus on sustainability. It is an apt metaphor for the current state of the world, I think.