Any internet forum, no matter its nationality, is subject to moderation by the admin and appointed moderators of that forum. There is no enshrined freedom of speech when you are a guest of someone else's forum. Where you do have (and I will always support) freedom of speech is on your own webpage, hosted on your own servers using your own connection. In any other situation you are subject to censorship by your ISP, by your hosting company, your government... in fact a great many people. Your ISP's terms and conditions, for example, will probably include something like "agree not to transmit or cause to be transmitted obscene or illegal material".
As for Zerogain's policy regarding arguments from elsewhere, the reasons for that are many. You have a personal disagreement with someone else that comes to some sort of conclusion elsewhere. Bringing it here makes the forum seem unpleasant and argumentative unecessarily. If you want to sort out a personal disagreement with an individual, talk to them.
Zerogain has been subject to legal threats in the past and I've no doubt will be again in the future. Bringing arguments from elsewhere can lead to the threat of lawsuits which is a burden that admin are quite happy to do without. Since no sort of subscription is demanded by Zerogain, it doesn't seem unreasonable to hope that people might abide by the policies of the site.