The manifesto is a window into the mind of the killer. It's a distasteful document, but a strong culture that trusts its people would encourage citizens to read it and understand the danger.
As it stands, authorities really don't trust people and instead prefer to nanny them by deflecting from any issues that lurk below the surface for fear of empowering another lone attacker.
This is a poor strategy because it will surely backfire. Instead, frank discussion is most conducive to lasting peace. But after seeing the response of the NZ government I hold little hope of that happening.
I think the American legal stance on this is right - it is not up to governments or people like you to decide what media other people consume, just because you fear they might be negatively influenced.