Forums exist for the communities they serve in this case students. The Forum is allowed to define what it considers acceptable and does have a duty to enforce the rules it has decided to implement.
The idea that it should be a free for all has never been part of the forums reason to exist.
It's simple really, we sign up to the forum and we agree to the rules and at no point does it say the site allows freedom of speech (in fact if you read the term it says quite the opposite)
If people don't like this they don't have to post on the forum.
I agree in the real world it may be different but I can say if someone said what he said about the Jews in the company I work for they would be dismissed because again the company has defined what it considers acceptable.