Though I have no clue what involvement the parents of bullies had in this particular case, I do know that often times parents are quite aware their child is a bully, and do little to stop the behavior, in many cases egging it on. Another thought to your question is that maybe the parents would simply get sued because you can't really sue a child can you? I mean, he didn't say 'charge the parents with a crime'... just sue.
Bullies quite often (if not entirely) are children who have abusive parents, and are taking their pain out on their schoolmates because they don't know how to deal with it, or they don't know it's wrong. It makes sense that the parents should take responsibility.