That suffers from a selection effect. Since not many sites prioritize free speech and only allow things within some narrower region of the overton window to be discussed it follows that the more extreme positions get pushed to sites that allow more.
If, hypothetically, every site were tolerant then you wouldn't have that association.
Also note that even 8chan is still moderated, each sub-board has its own rules and there also are global rules. What it really enables is a diversity of rules, set by each sub-community. What people seem to want is for sub-communities not to be allowed to exist even though they're not illegal. And that seems pretty dangerous.
