It was probably cheaper to force or induce the closure of index sites that are done more as a love of community than of profits, not make nearly as much as the usenet providers themselves. These index sites, I'm sure, understand the legal costs and risks of continued operation if those options were presented to them. With the majority of them down and out of the way the providers themselves will see a reduction in revenues and may not put up a fight when they fall into legal pressure to close up shop.