It simply doesn't occur to them -- as far as they are aware, the kid is sitting quietly at the computer, researching metaphysical ephemera. The fact that other kids might be terrorizing her by making words appear on a screen in front of her for all to see (or even just for her), doesn't compute.
We think of bullying as a face-to-face confrontation. But the Internet has opened up a whole new venue where kids can hang out and form cliques. IM, TEXT to IM, SMS, and Chat rooms are all new concepts to us, but the kids have not known a time when they were not a powerful means of communication within a peer group.