They aren't copying off Rock Band. Its a little thing called Competition
Plus, the Makers of Rock Band - Harmonix - helped make the first two Guitar Hero games. They just wanted to try something different and get the upper hand.
Most people saw it coming. Most people knew that The next Guitar hero would include Drums (And in my opinion, The drums look better on GH4). Anyway. If Activision/Neversoft hadn't included something like Drums/Vocals in their next game, people would get bored of it being the same as before and buy Rock band instead.
I will be buying Gh4. The drums look a lot better. And they should be easier for me to use since I'm a drummer myself.