[dragonwriter]

> But even if it were really "harmful pseudoscience", is that really a reason to ban it?

It is really a reason to decide not to distribute it.

> And if so, then who's in charge of deciding what's harmful pseudoscience and what's legitimate content

Anyone who is asked to actively participate in the chain of distribution.

> and what happens when they're wrong?

If someone disagrees with their decision not to distribute, they seek some other distribution. Except when it is a decision to distribute and the content is harmful in a way which produces liability, their decision being “wrong” in any authoritative sense isn't really an issue, only disagreement.