To be quite honest it makes a lot of sense if you look at it in this way:
Not everyone likes Hard Rock Music or Heavy metal.
Maybe they will release 50's, 60's Rock music, maybe Guitar Hero: Jazz and Guitar Hero : Game Themes
There are far more musical styles they can tap - Not just different groups.
These styles may require radically different visuals and play styles from the main "Hard Rock" style they currently use to fit the music which would mean that DLCs wouldn't always be appropriate.
Maybe they could even add in a REAL story mode thats interesting and not just a filler between songs - not that I would recommend MSG4 like cut scenes! Other game modes might make it worth it too.
On the other hand if they don't take this route and just medium to hard rock all the time they are just ripping people off (unless they are pretty cheap compared to getting them all on DLC)
Playing : PC AOE, DiRT 2, Runes of Magic, Wings of Prey & Planetside 2
Wii U : Nintendoland, Super Mario U & Fifa 2013 demo
DS : Guitar Hero : On Tour
Formerly unknown as Vengi