well they dont have to be, but they can.
i mean:
- if the guy with the memory-changing sword touches them, he could have made them believe they were evil
- they also could have been evil and got good by the sword-guy and then changed back, like he did with ginjou
but still... why did the fullbring girl said: she doesnt want ichigos power... this wouldnt really fit into both thesis... only if they knew it from the beginning and were evil..