1. People are stupid. Really, really stupid. Especially when given the means to surround and reinforce themselves with other idiots. See for example - antivax and alt-med in general, chemtrails any number of ridiculous conspiracy theories that propagate through the web.
2. These people are not thinkers, they are not open to having their viewpoints challenged and reason will not move them from their course.
I agree, this whole area is massively subjective, I'm not saying I have a solution. But this black and white idea that censorship is always bad, and the notion that we have a functional marketplace of ideas which people re-evaluate based on reason is ... well it's a fantasy.