it may not be a question of legality at all. I myself was not involved in this, but I think a large part of the decision was a matter of ethics. TWL, as a member of the internet community, has a responsibility to keep the internet a safe place. Teaching people to hack does not do this. Especially on a forum environment, where only little bits of info are availble here and there, this would simply tell people who don't really know what they are doing how to do some damage. This is how "script-kiddies" are probably created. I myself don't want TWL giving information out on how to setup a DDoS attack or anything like that. It doesn't promote safety and it isn't educational. The reason its not a freedom of speech argument is because we are not the government and the 1st amendment really only applies to the government (read it sometime).

note: this is not official TWL policy, but just my individual viewpoint on where those who made this policy might be coming from and why they might be justified