If you don't want it then don't buy it, but the developer is under no obligation to add a specific feature that is in this case probably used by an extreme minority. There's obviously a lot of custom stuff going on in how it handles input (the guitar has more buttons now then the controller does) so they probably decided it wasn't worth the effort to support a feature so infrequently used or requested.
You're not willing to pay for the game, you're willing to pay for a different game.
Guitar Hero III allows the use of the controller and I believe the previous two games did as well.
And no, they're not under any obligation to do so themselves, but they have lost an otherwise guaranteed sale by not doing so.