tell me why

Most Forum has their own rules, it depends deeply who and the type of person who is/are controling it, their "solid framwork of complaint" had no guarantee that they will work for the complain, nor they will go to judge who is right and who is wrong, it is their forum, they had absolute right to do what ever they wanted.
It is illegal to make copy to display movie without licences or without the film producer agreement to display them, however, the rule only apply if the film producer act, and there are no law that provide a police officer could see this as a crime and arrest people because they post the link of a movie that is without copyright (Until or unless they have warrants and make investigation and show that they had evidence to proof that such illegal thing exsisted). As a forum, they definately should try to keep things inside the forum legal and clean, instead of letting people to download movies in a illegal manner. And also, the Hong Kong Customs and Excise Department (香港海關) are responsible to those illegal movies link and Intellutual Properties that is own by people and company oversea, when they receive the complain from oversea company that is about oversea film privacy, they will investigate and do necessary action to stop such illegal thing (I think they investigate without being ask, but anyway, this seems to be the way they work).
Wheather allowing illegal thing would reduce the number of user using their forum is an unknown factor, nobody would know what is going to happen even if such complain ever made, and the worst case is that, nothing would change and affect the number of user. And the way forum earns money are usually by providing advertisement, the advertisement company pay the forum for posting their advertisement in their forum, it is quiet usually to see that (or paid to the Forum Server Company, whatever the way they do that).

You can sue them in the court for violating your freedom of speech.